WebGPUPathTracer: Add support for SkinnedMeshes

[gkjohnson]

• Adds support for skinned meshes using "SkinnedMeshBVH"
• Adds a custom version of "applyBoneMatrix" that takes a vec4 to support normals + positions (I'll probably try to get this added back to three)

cc @TheBlek

[gkjohnson]

@TheBlek - one thing I noticed while working on this was what looks like a floating point difference between the megakernel and wavefront kernel approach. You can see that there are some black artifacts in the megakernel one that aren't in the wavefront image. Is there a different offset being applied after hit somehow? Any thoughts?

Megakernel

Wavefront

You can repro it by loading this glb model and adding it to the scene like so:

new GLTFLoader().loadAsync( MODEL_URL ).then( gltf => {
	gltf.scene.traverse( c => {
		if ( c.isSkinnedMesh ) {
			scene.add( new Mesh( c.geometry ) );
		}
	} );
	pathTracer.setScene( scene, camera );
} );

[TheBlek]

Using getVertexPosition each time bounds for a triangle is requested does feel inefficient. Each vertex will be requested at least once, and probably more than that. So they can be calculated once and cached. Worth noting somewhere, that further improvements could probably be made to build speed at cost of memory. Perhaps its worth adding a benchmark case for skinned mesh build speed?

[TheBlek]

Is this equivalent to resolvePrimitiveIndex? If so, perhaps its best to use that for consistency?