Fix Weight Paint display with Subdivision Surface modifier.

The flag used to be set by the now removed old weight paint coloring code.

source/blender/blenkernel/intern/DerivedMesh.c

@@ -2348,6 +2348,10 @@ static CustomDataMask object_get_datamask(const Depsgraph *depsgraph, Object *ob
 			mask |= CD_MASK_MLOOPCOL;
 		}
 
+		if (ob->mode & OB_MODE_WEIGHT_PAINT) {
+			mask |= CD_MASK_MDEFORMVERT;
+		}
+
 		if (ob->mode & OB_MODE_EDIT)
 			mask |= CD_MASK_MVERT_SKIN;
 	}