Sculpt: Raise pbvh->leaf_limit to 400 for dyntopo

Setting pbvh->leaf_limit (the max triangles per node)
too low results in lots of distinct GPU meshes, which
can be slow for even moderately sized sculpt meshes
(starts to be a problem around 100-150k triangles).

source/blender/blenkernel/intern/pbvh_bmesh.c

@@ -1884,7 +1884,7 @@ void BKE_pbvh_build_bmesh(PBVH *pbvh,
   pbvh->bm_log = log;
 
   /* TODO: choose leaf limit better */
-  pbvh->leaf_limit = 100;
+  pbvh->leaf_limit = 400;
 
   BKE_pbvh_update_bmesh_offsets(pbvh, cd_vert_node_offset, cd_face_node_offset);