Meshlet remove per-cluster data upload

[JMS55]

Objective

• Per-cluster (instance of a meshlet) data upload is ridiculously expensive in both CPU and GPU time (8 bytes per cluster, millions of clusters, you very quickly run into PCIE bandwidth maximums, and lots of CPU-side copies and malloc).
• We need to be uploading only per-instance/entity data. Anything else needs to be done on the GPU.

Solution

• Per instance, upload:
  • meshlet_instance_meshlet_counts_prefix_sum - An exclusive prefix sum over the count of how many clusters each instance has.
  • meshlet_instance_meshlet_slice_starts - The starting index of the meshlets for each instance within the meshlets buffer.
• A new fill_cluster_buffers pass once at the start of the frame has a thread per cluster, and finds its instance ID and meshlet ID via a binary search of meshlet_instance_meshlet_counts_prefix_sum to find what instance it belongs to, and then uses that plus meshlet_instance_meshlet_slice_starts to find what number meshlet within the instance it is. The shader then writes out the per-cluster instance/meshlet ID buffers for later passes to quickly read from.
• I've gone from 45 -> 180 FPS in my stress test scene, and saved ~30ms/frame of overall CPU/GPU time.

[IceSentry]

I don't fully understand everything, but the code works, doesn't affect anything unrelated and is way faster than main. So LGTM

[atlv24]

this is quite ingenious, it took me a bit to understand how this worked