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>

src/platform/SDL2Renderer.h

@@ -0,0 +1,182 @@
+// SDL2Renderer.h - OpenGL ES 2 rendering implementation for SDL2 backend
+// This file provides the actual rendering implementation for the SDL2 types
+// defined in SDL2Types.h. It handles:
+// - OpenGL ES 2 context management
+// - Shader compilation and management
+// - Texture loading (via stb_image)
+// - Font rendering (via stb_truetype)
+// - Framebuffer object management
+//
+// Part of the renderer abstraction layer
+
+#pragma once
+
+#ifdef MCRF_SDL2
+
+#include <string>
+#include <vector>
+#include <unordered_map>
+#include <memory>
+
+// Forward declarations to avoid SDL header in this header
+struct SDL_Window;
+typedef void* SDL_GLContext;
+
+namespace sf {
+
+// Forward declarations
+class RenderTarget;
+class Texture;
+class Shader;
+
+// =============================================================================
+// SDL2 Renderer - Singleton managing OpenGL ES 2 state
+// =============================================================================
+
+class SDL2Renderer {
+public:
+    // Singleton access
+    static SDL2Renderer& getInstance();
+
+    // Initialization/shutdown
+    bool init();         // Initialize SDL2 (call before window creation)
+    bool initGL();       // Initialize OpenGL resources (call after GL context exists)
+    void shutdown();
+    bool isInitialized() const { return initialized_; }
+    bool isGLInitialized() const { return glInitialized_; }
+
+    // Built-in shader programs
+    enum class ShaderType {
+        Shape,      // For RectangleShape, CircleShape, etc.
+        Sprite,     // For textured sprites
+        Text,       // For text rendering
+        Custom      // User-provided shaders
+    };
+
+    // Get a built-in shader program
+    unsigned int getShaderProgram(ShaderType type) const;
+
+    // Compile a custom shader program
+    unsigned int compileShader(const std::string& vertexSource, const std::string& fragmentSource);
+    void deleteShaderProgram(unsigned int programId);
+
+    // Texture management
+    unsigned int createTexture(unsigned int width, unsigned int height, const unsigned char* pixels = nullptr);
+    void updateTexture(unsigned int textureId, unsigned int x, unsigned int y,
+                       unsigned int width, unsigned int height, const unsigned char* pixels);
+    void deleteTexture(unsigned int textureId);
+    void setTextureSmooth(unsigned int textureId, bool smooth);
+    void setTextureRepeated(unsigned int textureId, bool repeated);
+
+    // FBO management
+    unsigned int createFBO(unsigned int width, unsigned int height, unsigned int& colorTexture);
+    void deleteFBO(unsigned int fboId);
+    void bindFBO(unsigned int fboId);
+    void unbindFBO();
+
+    // Render state management
+    void setViewport(int x, int y, unsigned int width, unsigned int height);
+    void setProjection(float left, float right, float bottom, float top);
+    void clear(float r, float g, float b, float a);
+
+    // Drawing primitives
+    void drawTriangles(const float* vertices, size_t vertexCount,
+                       const float* colors, const float* texCoords,
+                       unsigned int textureId = 0);
+
+    // Projection matrix access (for shaders)
+    const float* getProjectionMatrix() const { return projectionMatrix_; }
+
+private:
+    SDL2Renderer() = default;
+    ~SDL2Renderer() = default;
+    SDL2Renderer(const SDL2Renderer&) = delete;
+    SDL2Renderer& operator=(const SDL2Renderer&) = delete;
+
+    bool initialized_ = false;
+    bool glInitialized_ = false;
+
+    // Built-in shader programs
+    unsigned int shapeProgram_ = 0;
+    unsigned int spriteProgram_ = 0;
+    unsigned int textProgram_ = 0;
+
+    // Current projection matrix (4x4 orthographic)
+    float projectionMatrix_[16] = {0};
+
+    // FBO stack for nested render-to-texture
+    std::vector<unsigned int> fboStack_;
+
+    // Helper functions
+    bool compileAndLinkProgram(const char* vertexSrc, const char* fragmentSrc, unsigned int& programOut);
+    unsigned int compileShaderStage(unsigned int type, const char* source);
+    void initBuiltinShaders();
+};
+
+// =============================================================================
+// Keyboard/Mouse SDL2 Implementation helpers
+// =============================================================================
+
+// SDL2 scancode to sf::Keyboard::Key mapping
+int sdlScancodeToSfKey(int sdlScancode);
+int sfKeyToSdlScancode(int sfKey);
+
+// SDL2 mouse button to sf::Mouse::Button mapping
+int sdlButtonToSfButton(int sdlButton);
+int sfButtonToSdlButton(int sfButton);
+
+// =============================================================================
+// Event translation
+// =============================================================================
+
+// Translate SDL_Event to sf::Event
+// Returns true if the event was translated, false if it should be ignored
+bool translateSDLEvent(const void* sdlEvent, void* sfEvent);
+
+// =============================================================================
+// Font Atlas for text rendering
+// =============================================================================
+
+class FontAtlas {
+public:
+    FontAtlas();
+    ~FontAtlas();
+
+    // Load font data
+    bool load(const unsigned char* fontData, size_t dataSize, float fontSize);
+
+    // Get texture atlas
+    unsigned int getTextureId() const { return textureId_; }
+
+    // Get glyph info for rendering
+    struct GlyphInfo {
+        float u0, v0, u1, v1;   // Texture coordinates
+        float xoff, yoff;        // Offset from cursor
+        float xadvance;          // How far to advance cursor
+        float width, height;     // Glyph dimensions in pixels
+    };
+
+    bool getGlyph(uint32_t codepoint, GlyphInfo& info) const;
+
+    // Get font metrics
+    float getAscent() const { return ascent_; }
+    float getDescent() const { return descent_; }
+    float getLineHeight() const { return lineHeight_; }
+
+private:
+    unsigned int textureId_ = 0;
+    float fontSize_ = 0;
+    float ascent_ = 0;
+    float descent_ = 0;
+    float lineHeight_ = 0;
+
+    // Glyph cache - maps codepoint to glyph info
+    std::unordered_map<uint32_t, GlyphInfo> glyphCache_;
+
+    // stb_truetype font info (opaque pointer to avoid header inclusion)
+    void* stbFontInfo_ = nullptr;
+};
+
+} // namespace sf
+
+#endif // MCRF_SDL2