Implement hierarchical z-buffer for CPU occlusion queries

[adriengivry]

Problem this feature should fix

Outside of frustum culling, Overload doesn't provide any other culling strategy.
One that could be really beneficial is occlusion query.

Expected solution

• It could be done with the GPU, but would require synchronization
• or it could be down on the CPU, using a simple rasterizer

Essentially, we could generate "inbouds" for geometry (simple cubes that fits inside of non-transparent geometry). And render these cubes into a low-resolution buffer using a software rasterizer. Then, before drawing any geometry, its bounds projected onto the screen would be tested against the rasterization output.