Reduce OM limit for sculpting after discussion with Antony

That's rather tricky to think of a good threshold here, and
maybe we'd better use something based on the number of faces
instead.

Anyway, let's give it a try this way and see what happens.

source/blender/editors/sculpt_paint/sculpt_intern.h

@@ -134,7 +134,7 @@ void sculpt_update_object_bounding_box(struct Object *ob);
 #ifdef DEBUG
 #  define SCULPT_OMP_LIMIT 0
 #else
-#  define SCULPT_OMP_LIMIT 8
+#  define SCULPT_OMP_LIMIT 4
 #endif
 
 #endif