Sculpt: Use vertices instead of faces to limit the grids in each PBVH node

This uses the vertices per grid instead of quads to set the limit of
grids per PBVH Node. This should create more leaf nodes in lower
subdivisions levels where the duplicates count is high, producing more
uniform performance across different levels.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8454

source/blender/blenkernel/intern/pbvh.c

@@ -644,7 +644,7 @@ void BKE_pbvh_build_grids(PBVH *pbvh,
   pbvh->totgrid = totgrid;
   pbvh->gridkey = *key;
   pbvh->grid_hidden = grid_hidden;
-  pbvh->leaf_limit = max_ii(LEAF_LIMIT / ((gridsize - 1) * (gridsize - 1)), 1);
+  pbvh->leaf_limit = max_ii(LEAF_LIMIT / (gridsize * gridsize), 1);
 
   BB cb;
   BB_reset(&cb);