Performance: splat culling

[aras-p]

Currently all splats are rendered (and sorted), just quads that are behind camera are thrown into the sea at vertex shader level.

It should be possible to do some sort of splat culling at distance evaluation state. Both "behind camera" and "really outside of the frustum" type. Either at individual splat level, or whole splat chunk (groups of 256 splats) level.

And some sort of GPU stream compaction so that later on fewer splats need to be sorted, and fewer quads need to be drawn. Both sorting and drawing would need to get changed to do indirect dispatch/draw.