Function createInstancedGSplatsMesh

Create an instanced mesh for gsplats rendering.

Sets up the instanced geometry with all per-splat attributes.

Note: We use THREE.Mesh instead of THREE.InstancedMesh because:

• InstancedMesh adds instanceMatrix (mat4 = 4 attribute locations)
• Our shader computes positions from custom attributes, not matrices
• This avoids exceeding WebGL's 16 attribute location limit
• InstancedBufferGeometry with Mesh still uses instanced drawing