Draw Cache: extract tris in parallel ranges

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

1 files changed, 3 insertions, 1 deletions

diff --git a/source/blender/draw/intern/draw_cache_impl_mesh.c b/source/blender/draw/intern/draw_cache_impl_mesh.c
index 8387fb2f43f..38ecdf7756b 100644
--- a/source/blender/draw/intern/draw_cache_impl_mesh.c
+++ b/source/blender/draw/intern/draw_cache_impl_mesh.c
@@ -856,7 +856,9 @@ static void mesh_buffer_extraction_cache_clear(MeshBufferExtractionCache *extrac
   extraction_cache->loose_geom.edge_len = 0;
   extraction_cache->loose_geom.vert_len = 0;
 
-  MEM_SAFE_FREE(extraction_cache->mat_offsets.tri);
+  MEM_SAFE_FREE(extraction_cache->poly_sorted.tri_first_index);
+  MEM_SAFE_FREE(extraction_cache->poly_sorted.mat_tri_len);
+  extraction_cache->poly_sorted.visible_tri_len = 0;
 }
 
 static void mesh_batch_cache_clear(Mesh *me)