- Added RenderTexture support to UIDrawable base class
- std::unique_ptr<sf::RenderTexture> for opt-in rendering
- Dirty flag system for optimization
- enableRenderTexture() and markDirty() methods
- Implemented clip_children property for UIFrame
- Python-accessible boolean property
- Automatic RenderTexture creation when enabled
- Proper coordinate transformation for nested frames
- Updated UIFrame::render() for clipping support
- Renders to RenderTexture when clip_children=true
- Handles nested clipping correctly
- Only re-renders when dirty flag is set
- Added comprehensive dirty flag propagation
- All property setters mark frame as dirty
- Size changes recreate RenderTexture
- Animation system integration
- Created tests for clipping functionality
- Basic clipping test with visual verification
- Advanced nested clipping test
- Dynamic resize handling test
This is Phase 1 of the RenderTexture overhaul, providing the foundation
for advanced rendering effects like blur, glow, and viewport rendering.
