feat(webgl): Replace legacy GL with modern OpenGL ES 3.0

Major refactoring to remove ALL legacy OpenGL immediate mode calls from
WEBGL_GAL and replace them with WebGL 2.0/OpenGL ES 3.0 compatible code.

Key changes:
- Add MVP matrix uniform support (replaces glMatrixMode/glOrtho/glLoadMatrix)
- Add matrix math helpers (computeOrthoMatrix, multiplyMatrix4x4, etc.)
- Replace glBegin/glEnd/glVertex with vertex manager pattern
- Rewrite DrawCursor to use vertex manager and DrawLine() calls
- Rewrite DrawBitmap to use vertex manager with SHADER_FONT mode
- Replace glEnableClientState with glVertexAttribPointer
- Replace glDrawBuffer with glDrawBuffers (WebGL 2.0 API)
- Add fullscreen_quad.cpp/h for compositor Present()
- Update SHADER class with mat4 uniform support
- Add MultiplyMatrix() to vertex_manager for Transform()
- Remove all legacy GL stubs from wasm_stubs.cpp

Build now completes with FULL_ES3 mode (no LEGACY_GL_EMULATION).
Remaining issue: wxWidgets GL library calls glColor3f internally.

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

scripts/build-gal-webgl-test.sh

@@ -61,6 +61,11 @@ fi
 # (Makefile only tracks object file dependencies, not linker flag changes)
 rm -f "$OUTPUT_DIR"/*.js "$OUTPUT_DIR"/*.wasm 2>/dev/null || true
 
+# Generate shaders (converts GLSL 1.20 to GLSL ES 3.00)
+echo ""
+echo "Generating WebGL shaders..."
+python3 generate_shaders.py
+
 echo ""
 echo "Building..."
 if [ "$DEBUG_BUILD" = "1" ]; then