Shader POC: Add shader_enabled property to UIFrame (#106)

Proof of concept for shader support on UIFrame:
- Add shader and shader_enabled members to UIFrame
- Add initializeTestShader() with hardcoded wave/glow fragment shader
- Add shader_enabled Python property for toggling
- Apply shader when drawing RenderTexture sprite
- Auto-update time uniform for animated effects

Also fixes position corruption when toggling RenderTexture usage:
- Standard rendering path now uses `position` as source of truth
- Prevents box position from staying at (0,0) after texture render

Test files:
- tests/shader_poc_test.py: Visual test of 6 render variants
- tests/shader_toggle_test.py: Regression test for position bug

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

tests/shader_poc_test.py

@@ -0,0 +1,132 @@
+#!/usr/bin/env python3
+"""
+Shader POC Test - Issue #106
+
+Tests 6 render variants:
+1. Basic frame (no special flags)
+2. clip_children=True
+3. cache_subtree=True
+4. Basic + shader_enabled
+5. clip_children + shader_enabled
+6. cache_subtree + shader_enabled
+
+The shader applies a wave distortion + glow effect.
+Shader-enabled frames should show visible animation.
+"""
+import mcrfpy
+import sys
+
+# Create test scene
+scene = mcrfpy.Scene("shader_test")
+mcrfpy.current_scene = scene
+ui = scene.children
+
+# Create a background
+bg = mcrfpy.Frame(pos=(0, 0), size=(1024, 768), fill_color=(30, 30, 40, 255))
+ui.append(bg)
+
+# Helper to create a test frame with children
+def create_test_frame(x, y, label, clip=False, cache=False, shader=False):
+    """Create a frame with some child content for testing."""
+    frame = mcrfpy.Frame(
+        pos=(x, y),
+        size=(200, 200),
+        fill_color=(60, 80, 120, 255),
+        outline_color=(200, 200, 255, 255),
+        outline=2.0,
+        clip_children=clip,
+        cache_subtree=cache
+    )
+
+    # Add some child content
+    title = mcrfpy.Caption(text=label, pos=(5, 5), font_size=12)
+    title.fill_color = (255, 255, 200, 255)
+    frame.children.append(title)
+
+    # Add a sprite or shape inside
+    inner = mcrfpy.Frame(
+        pos=(20, 35),
+        size=(110, 60),
+        fill_color=(100, 150, 200, 255),
+        outline_color=(255, 255, 255, 200),
+        outline=1.0
+    )
+    frame.children.append(inner)
+
+    # Add text inside inner frame
+    inner_text = mcrfpy.Caption(text="Content", pos=(25, 20), font_size=14)
+    inner_text.fill_color = (255, 255, 255, 255)
+    inner.children.append(inner_text)
+
+    # Enable shader if requested
+    if shader:
+        frame.shader_enabled = True
+
+    return frame
+
+# Row 1: Without shader
+y1 = 50
+title1 = mcrfpy.Caption(text="Without Shader:", pos=(20, y1 - 30), font_size=16)
+title1.fill_color = (255, 255, 100, 255)
+ui.append(title1)
+
+# 1. Basic (no flags)
+basic = create_test_frame(50, y1, "Basic", clip=False, cache=False, shader=False)
+ui.append(basic)
+
+# 2. clip_children
+clipped = create_test_frame(300, y1, "clip_children", clip=True, cache=False, shader=False)
+ui.append(clipped)
+
+# 3. cache_subtree
+cached = create_test_frame(550, y1, "cache_subtree", clip=False, cache=True, shader=False)
+ui.append(cached)
+
+# Row 2: With shader
+y2 = 300
+title2 = mcrfpy.Caption(text="With Shader (should animate):", pos=(20, y2 - 30), font_size=16)
+title2.fill_color = (255, 255, 100, 255)
+ui.append(title2)
+
+# 4. Basic + shader
+basic_shader = create_test_frame(50, y2, "Basic+Shader", clip=False, cache=False, shader=True)
+ui.append(basic_shader)
+
+# 5. clip_children + shader
+clipped_shader = create_test_frame(300, y2, "clip+Shader", clip=True, cache=False, shader=True)
+ui.append(clipped_shader)
+
+# 6. cache_subtree + shader
+cached_shader = create_test_frame(550, y2, "cache+Shader", clip=False, cache=True, shader=True)
+ui.append(cached_shader)
+
+# Add instructions
+#instructions = mcrfpy.Caption(
+#    text="Press Q or Escape to quit. Bottom row should show animated wave/glow effects.",
+#    pos=(20, 400),
+#    font_size=14
+#)
+#instructions.fill_color = (180, 180, 180, 255)
+#ui.append(instructions)
+
+# Debug info
+#debug_info = mcrfpy.Caption(
+#    text=f"Frames created: 6 variants (3 without shader, 3 with shader)",
+#    pos=(20, 430),
+#    font_size=12
+#)
+#debug_info.fill_color = (120, 120, 120, 255)
+#ui.append(debug_info)
+
+# Keyboard handler
+def on_key(key, state):
+    if state == "start" and key in ("Q", "Escape"):
+        print("PASS: Shader POC test complete - exiting")
+        sys.exit(0)
+
+scene.on_key = on_key
+
+print("Shader POC Test running...")
+print("- Top row: No shader (static)")
+print("- Bottom row: Shader enabled (should animate with wave/glow)")
+print("Press Q or Escape to quit")