Blender Git Loki

Cycles: add shadow path compaction for GPU rendering

Similar to main path compaction that happens before adding work tiles, this
compacts shadow paths before launching kernels that may add shadow paths.

Only do it when more than 50% of space is wasted.

It's not a clear win in all scenes, some are up to 1.5% slower. Likely caused
by different order of scheduling kernels having an unpredictable performance
impact. Still feels like compaction is just the right thing to avoid cases
where a few shadow paths can hold up a lot of main paths.

Differential Revision: https://developer.blender.org/D12944