WebGPUPathTracer: Add support for opacity during BVH traversal

[gkjohnson]

• Add a 1D PCG random function
• Merge the "hit" and "ray" queues to free up a storage buffer slot
• Check the opacity of the object material on triangle intersection, continue traversal if transparent
• Skip traversal of the object entirely if opacity is set to 0

This should reduce the number of raycasts required for transparent surfaces.

Future

• Sample and include albedo and alpha map opacity
• We could handle triangle sided-ness this way, as well?

cc @TheBlek