Add heatmap debug mode for compute GSplat rasterizer

[mvaligursky]

Adds a new GSPLAT_DEBUG_HEATMAP debug rendering mode for the compute Gaussian splat rasterizer. The heatmap visualizes the average number of splats processed per pixel in each tile before the transmittance early-out, displayed as a blue-to-red color ramp at tile resolution.

Changes:

• Add GSPLAT_DEBUG_HEATMAP constant for the new debug mode
• Implement tile-averaged splat count visualization in the compute rasterize shader using workgroup-shared atomics
• Wire heatmap mode through the dispatch set and compute renderer as a shader variant
• Add Heatmap option to the debug dropdown in all GSplat examples

API Changes:

• New constant: GSPLAT_DEBUG_HEATMAP — set via app.scene.gsplat.debug = GSPLAT_DEBUG_HEATMAP
• Only supported with GSPLAT_RENDERER_COMPUTE