Implement per-layer dirty tracking and RenderTexture caching for
ColorLayer and TileLayer. Each layer now maintains its own cached
texture and only re-renders when content changes.
Key changes:
- Add dirty flag, cached_texture, and cached_sprite to GridLayer base
- Implement renderToTexture() for both ColorLayer and TileLayer
- Mark layers dirty on: set(), fill(), resize(), texture change
- Viewport changes (center/zoom) just blit cached texture portion
- Fallback to direct rendering if texture creation fails
- Add regression test with performance benchmarks
Expected performance improvement: Static layers render once, then
viewport panning/zooming only requires texture blitting instead of
re-rendering all cells.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
