Add SDL2+OpenGL ES 2 renderer backend for Emscripten/WebGL

Implements Phase 1 of renderer abstraction plan:
- SDL2Types.h: SFML-compatible type stubs in sf:: namespace
- SDL2Renderer.h/cpp: OpenGL ES 2 rendering implementation
- EmscriptenStubs.cpp: Stubs for missing POSIX functions (wait3, wait4, wcsftime)

Build system changes:
- Add MCRF_SDL2 compile-time backend selection
- Add Emscripten SDL2 link options (-sUSE_SDL=2, -sFULL_ES2=1)
- Fix LONG_BIT mismatch for Emscripten in pyport.h

Code changes for SDL2/headless compatibility:
- Guard ImGui includes with !MCRF_HEADLESS && !MCRF_SDL2
- Defer GL shader initialization until after context creation

Current status: Python runs in browser, rendering WIP (canvas sizing issues)

Build commands:
  emcmake cmake -DMCRF_SDL2=ON -B build-emscripten
  emmake make -C build-emscripten

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CMakeLists.txt

@@ -11,10 +11,22 @@ set(CMAKE_CXX_STANDARD_REQUIRED True)
 # Headless build option (no SFML, no graphics - for server/testing/Emscripten prep)
 option(MCRF_HEADLESS "Build without graphics dependencies (SFML, ImGui)" OFF)
 
-# Emscripten builds are always headless (no SFML yet - using stubs)
+# SDL2 backend option (SDL2 + OpenGL ES 2 - for Emscripten/WebGL, Android, cross-platform)
+option(MCRF_SDL2 "Build with SDL2+OpenGL ES 2 backend instead of SFML" OFF)
+
+# Emscripten builds: use SDL2 if specified, otherwise fall back to headless
 if(EMSCRIPTEN)
-    set(MCRF_HEADLESS ON)
-    message(STATUS "Emscripten detected - forcing HEADLESS mode")
+    if(MCRF_SDL2)
+        message(STATUS "Emscripten detected - using SDL2 backend")
+        set(MCRF_HEADLESS OFF)
+    else()
+        set(MCRF_HEADLESS ON)
+        message(STATUS "Emscripten detected - forcing HEADLESS mode (use -DMCRF_SDL2=ON for graphics)")
+    endif()
+endif()
+
+if(MCRF_SDL2)
+    message(STATUS "Building with SDL2 backend - SDL2+OpenGL ES 2")
 endif()
 
 if(MCRF_HEADLESS)
@@ -36,9 +48,13 @@ if(EMSCRIPTEN)
     # Emscripten build: use Python headers compiled for wasm32-emscripten
     # The pyconfig.h from cross-build has correct LONG_BIT and other settings
     set(PYTHON_WASM_BUILD "${CMAKE_SOURCE_DIR}/deps/cpython/cross-build/wasm32-emscripten/build/python")
+    # Force-include wasm pyconfig.h BEFORE anything else to set correct platform defines
     add_compile_options(-include ${PYTHON_WASM_BUILD}/pyconfig.h)
+    # Override LONG_BIT - Emscripten's limits.h incorrectly defines it as 64 for wasm32
+    add_compile_definitions(LONG_BIT=32)
+    # Include wasm build directory FIRST so its pyconfig.h is found by #include "pyconfig.h"
+    include_directories(BEFORE ${PYTHON_WASM_BUILD})
     include_directories(${CMAKE_SOURCE_DIR}/deps/cpython/Include)
-    include_directories(${PYTHON_WASM_BUILD})  # For generated headers
     message(STATUS "Using Emscripten Python from: ${PYTHON_WASM_BUILD}")
 elseif(MCRF_CROSS_WINDOWS)
     # Windows cross-compilation: use cpython headers with PC/pyconfig.h
@@ -57,8 +73,9 @@ else()
     include_directories(${CMAKE_SOURCE_DIR}/deps/Python)
 endif()
 
-# ImGui and ImGui-SFML include directories (not needed in headless mode)
-if(NOT MCRF_HEADLESS)
+# ImGui and ImGui-SFML include directories (not needed in headless or SDL2 mode)
+# SDL2 builds will use ImGui with SDL2 backend later; for now, no ImGui
+if(NOT MCRF_HEADLESS AND NOT MCRF_SDL2)
     include_directories(${CMAKE_SOURCE_DIR}/modules/imgui)
     include_directories(${CMAKE_SOURCE_DIR}/modules/imgui-sfml)
 
@@ -75,13 +92,14 @@ endif()
 # Collect all the source files
 file(GLOB_RECURSE SOURCES "src/*.cpp")
 
-# Add ImGui sources to the build (only if not headless)
-if(NOT MCRF_HEADLESS)
+# Add ImGui sources to the build (only if using SFML)
+if(NOT MCRF_HEADLESS AND NOT MCRF_SDL2)
     list(APPEND SOURCES ${IMGUI_SOURCES})
 endif()
 
 # Find OpenGL (required by ImGui-SFML) - not needed in headless mode
-if(NOT MCRF_HEADLESS)
+# SDL2 builds handle OpenGL ES 2 differently (via SDL2 or Emscripten)
+if(NOT MCRF_HEADLESS AND NOT MCRF_SDL2)
     if(MCRF_CROSS_WINDOWS)
         # For cross-compilation, OpenGL is provided by MinGW
         set(OPENGL_LIBRARIES opengl32)
@@ -109,8 +127,27 @@ if(EMSCRIPTEN)
         ${LIBTCOD_WASM_BUILD}/_deps/lodepng-c-build/liblodepng-c.a
         ${LIBTCOD_WASM_BUILD}/_deps/utf8proc-build/libutf8proc.a)
     include_directories(${CMAKE_SOURCE_DIR}/deps/platform/linux)  # Use Linux platform stubs for now
+    # For SDL2 builds, add stb headers for image/font loading
+    if(MCRF_SDL2)
+        include_directories(${CMAKE_SOURCE_DIR}/deps/stb)
+    endif()
     message(STATUS "Linking Emscripten Python: ${PYTHON_WASM_BUILD}/libpython3.14.a")
     message(STATUS "Linking Emscripten libtcod: ${LIBTCOD_WASM_BUILD}/libtcod.a")
+elseif(MCRF_SDL2)
+    # SDL2 build (non-Emscripten): link against SDL2 and system libraries
+    # Note: For desktop SDL2 builds in the future
+    find_package(SDL2 REQUIRED)
+    find_package(OpenGL REQUIRED)
+    set(LINK_LIBS
+        SDL2::SDL2
+        OpenGL::GL
+        tcod
+        python3.14
+        m dl util pthread)
+    include_directories(${CMAKE_SOURCE_DIR}/deps/platform/linux)
+    include_directories(${CMAKE_SOURCE_DIR}/deps/stb)  # stb_image.h, stb_truetype.h
+    link_directories(${CMAKE_SOURCE_DIR}/__lib)
+    message(STATUS "Building with SDL2 backend (desktop)")
 elseif(MCRF_HEADLESS)
     # Headless build: no SFML, no OpenGL
     if(WIN32 OR MCRF_CROSS_WINDOWS)
@@ -195,6 +232,8 @@ endif()
 add_executable(mcrogueface ${SOURCES})
 
 # Define NO_SDL for libtcod-headless headers (excludes SDL-dependent code)
+# We ALWAYS need this because libtcod headers expect SDL3, not SDL2
+# Our SDL2 backend is separate from libtcod's SDL3 renderer
 target_compile_definitions(mcrogueface PRIVATE NO_SDL)
 
 # Define MCRF_HEADLESS for headless builds (excludes SFML/ImGui code)
@@ -202,9 +241,15 @@ if(MCRF_HEADLESS)
     target_compile_definitions(mcrogueface PRIVATE MCRF_HEADLESS)
 endif()
 
+# Define MCRF_SDL2 for SDL2 builds (uses SDL2+OpenGL ES 2 instead of SFML)
+if(MCRF_SDL2)
+    target_compile_definitions(mcrogueface PRIVATE MCRF_SDL2)
+endif()
+
 # Emscripten-specific link options (use ports for zlib, bzip2, sqlite3)
 if(EMSCRIPTEN)
-    target_link_options(mcrogueface PRIVATE
+    # Base Emscripten options
+    set(EMSCRIPTEN_LINK_OPTIONS
         -sUSE_ZLIB=1
         -sUSE_BZIP2=1
         -sUSE_SQLITE3=1
@@ -215,6 +260,9 @@ if(EMSCRIPTEN)
         -sSTACK_OVERFLOW_CHECK=2
         -fexceptions
         -sNO_DISABLE_EXCEPTION_CATCHING
+        # Disable features that require dynamic linking support
+        -sERROR_ON_UNDEFINED_SYMBOLS=0
+        -sALLOW_UNIMPLEMENTED_SYSCALLS=1
         # Preload Python stdlib into virtual filesystem at /lib/python3.14
         --preload-file=${CMAKE_SOURCE_DIR}/wasm_stdlib/lib@/lib
         # Preload game scripts into /scripts
@@ -222,6 +270,24 @@ if(EMSCRIPTEN)
         # Preload assets
         --preload-file=${CMAKE_SOURCE_DIR}/assets@/assets
     )
+
+    # Add SDL2 options if using SDL2 backend
+    if(MCRF_SDL2)
+        list(APPEND EMSCRIPTEN_LINK_OPTIONS
+            -sUSE_SDL=2
+            -sFULL_ES2=1
+            -sMIN_WEBGL_VERSION=2
+            -sMAX_WEBGL_VERSION=2
+        )
+        # SDL2 flags are also needed at compile time for headers
+        target_compile_options(mcrogueface PRIVATE
+            -sUSE_SDL=2
+        )
+        message(STATUS "Emscripten SDL2 options enabled: -sUSE_SDL=2 -sFULL_ES2=1")
+    endif()
+
+    target_link_options(mcrogueface PRIVATE ${EMSCRIPTEN_LINK_OPTIONS})
+
     # Set Python home for the embedded interpreter
     target_compile_definitions(mcrogueface PRIVATE
         MCRF_WASM_PYTHON_HOME="/lib/python3.14"