Add comprehensive visual regression testing using Makepad's test framework:
Makepad Test App:
- Create makepad_test_app with 13 test scenarios
- Test empty map, single/multiple tiles, zoom levels 10-16
- Test dark theme, POIs, labels, roads, water, buildings
- Support both interactive and headless modes
- Include UPDATE_GOLDEN environment variable for updating references
Test Infrastructure:
- Add MAKEPAD_TESTING_GUIDE.md with comprehensive documentation
- Create visual_regression.rs with 13 test functions
- Add screenshot comparison with 1% threshold
- Save diff images on failure for debugging
- Support golden image directory structure
Test Scenarios:
1. empty_map - No tiles loaded
2. single_tile_amsterdam - Single tile render
3. multiple_tiles_grid - 3x3 tile grid
4. zoom_level_10 - Low zoom overview
5. zoom_level_12 - Medium zoom
6. zoom_level_14 - Standard city zoom
7. zoom_level_16 - High zoom with details
8. dark_theme - Dark theme rendering
9. pois_visible - Point of interest icons
10. labels_visible - Text labels
11. roads_render - Road geometry
12. water_features - Water/canal rendering
13. buildings_render - Building footprints
This completes the visual testing component of Phase 6.
