Replace full-screen quad composite with tile-based indirect draw in local compute GSplat renderer

[mvaligursky]

Replace the full-screen quad composite blit in the local compute GSplat renderer with a tile-based indirect draw that only composites non-empty tiles. This fixes a stale data bug where tiles with zero splats retained data from previous frames, and reduces fill rate proportional to empty screen area.

Changes:

• Add GSplatTileComposite class that owns a bare mesh (no vertex/index buffers), a ShaderMaterial, and a MeshInstance configured for non-indexed indirect draw
• Add gsplatTileCompositeVS WGSL vertex shader chunk that procedurally generates tile quads from pcVertexIndex and a rasterizeTileList storage buffer
• Extend the classify compute shader to write DrawIndirectArgs for the tile composite alongside existing indirect dispatch args
• Remove the old full-screen quad composite (_createCompositeMaterial, _createMeshInstance) from GSplatComputeLocalRenderer

Performance:

• Saves fill rate proportional to empty screen area — only tiles containing splats are composited
• No additional GPU passes — the classify shader already has rasterizeCount and writes draw args in the same thread-0 block
• No vertex or index buffers needed — quads generated procedurally in the vertex shader