Provide a CompoundMeshBVH class

[gkjohnson]

Somewhat related to #388 about providing a TLAS for multiple objects at the scene level but this breaks down if the bounding boxes of the leaf objects are too large and have significant overlap. This ultimlately results in all BLAS being traversed anyway leading to no improvement.

This compound BVH could leverage the "indirect buffer" improvements made in #555 and index into multiple child meshes to avoid the issue with parent bounds overlapping.

• Assumes meshes are roughly static, though refit is still possible.
• Requires that geometry references do not change.
• All vertices would need to be transformed by world matrices.
• Would be easiest to implement with an abstraction that #243 and #388 would need.
• Would require reserving some high bits for mesh index and low bits for triangle index or a separate buffer for mesh indices vs triangle index. (ie 10-14 bits for meshes, 22-18 bits for triangles). Could by dynamically computed by apparent needs.