radv/bvh: Add custom leaf node builder

This custom builder implements fine-grained instance node bounds
calculation by looking at all AABBs at tree depth 2.

Shaves off 0.3ms in the start scene for Indiana Jones: The Great Circle
on Deck (roughly 29.1ms->28.7ms).

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32797>
3 files changed