Blender Git Loki

Cleanup: Move 'tris_per_mat' member out of 'MeshBufferCache'

`MeshBufferCache` is a struct representing a list of buffers.

As such, `GPUIndexBuf **tris_per_mat` is out of place as it does not
represent one of the buffers in the list.

In fact this member should be close to `GPUBatch **surface_per_mat` as
they are related.

The code for dependencies between buffer and batch had to be reworked
as it relies on the member's position.

Differential Revision: https://developer.blender.org/D12227