Sculpt: Make Sculpt Vertex Colors features experimental

This disables all Sculpt Vertex Colors tools, operators, panels and rendering capabilities and puts them under the "Use Sculpt Vertex Colors" experimental option.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8239

source/blender/blenkernel/intern/paint.c

@@ -1514,7 +1514,7 @@ static void sculpt_update_object(Depsgraph *depsgraph,
 
   /* Add a color layer if a color tool is used. */
   Mesh *orig_me = BKE_object_get_original_mesh(ob);
-  if (need_colors) {
+  if (need_colors && U.experimental.use_sculpt_vertex_colors) {
     if (!CustomData_has_layer(&orig_me->vdata, CD_PROP_COLOR)) {
       CustomData_add_layer(&orig_me->vdata, CD_PROP_COLOR, CD_DEFAULT, NULL, orig_me->totvert);
       BKE_mesh_update_customdata_pointers(orig_me, true);