Textures: Optimize textures by grouping per tile

[bertt]

Currently, multiple texture files can exist per feature within a tile. For example, in the Delfshaven sample, a single building has 18 JPG textures totaling 2.48 MB.

Improvement proposal

• Implement texture grouping per tile, so all features in a tile share a single combined texture.
• Benefits:
  • Smaller GLTF/GLB sizes
  • Fewer texture files per tile
  • Improved rendering performance