The `ibo.tris` extraction in multithread is currently only done if the
mesh has only 1 material.
Now we cache a map indicating the index of each polygon after sort and
thus allow the extraction of tris with materials in multithreaded.
As caching is a heavy operation and was already being performed in
multi-thread for triangle offsets, no significant improvements are
expected.
The benefit will be much greater when we can skip updating the cache
while transforming a geometry.
**Profiling:**
||master:|PATCH:
|---|---|---|
|large_mesh_editing_materials:|Average: 13.855380 FPS|Average: 15.525684 FPS
||rdata 9ms iter 36ms (frame 71ms)|rdata 9ms iter 29ms (frame 64ms)
|subdiv_mesh_final_only_materials:|Average: 28.113742 FPS|Average: 28.633599 FPS
||rdata 0ms iter 1ms (frame 36ms)|rdata 0ms iter 1ms (frame 35ms)
1.1x overall speedup
Differential Revision: https://developer.blender.org/D11445
178086d581