Multires: Promote OSD CCG to be officially supported

Only active when WITH_OPENSUBDIV_MODIFIER is set to ON.

There is still some work to be done to make that one enabled by
default.

source/blender/modifiers/intern/MOD_multires.c

@@ -228,16 +228,12 @@ static Mesh *applyModifier_subdiv(ModifierData *md,
 		/* Happens on bad topology, ut also on empty input mesh. */
 		return result;
 	}
-	/* TODO(sergey): Some of production machines are using OpenSubdiv already.
-	 * so better not enable semi-finished multires sculpting for now. Will give
-	 * a wrong impression that things do work, even though crucial areas are
-	 * still missing in implementation.
+	/* NOTE: Orco needs final coordinates on CPU side, which are expected to be
+	 * accessible via MVert. For this reason we do not evaluate multires to
+	 * grids when orco is requested.
 	 */
 	const bool for_orco = (ctx->flag & MOD_APPLY_ORCO) != 0;
-	if ((ctx->object->mode & OB_MODE_SCULPT) &&
-	    G.debug_value == 128 &&
-	    !for_orco)
-	{
+	if ((ctx->object->mode & OB_MODE_SCULPT) && !for_orco) {
 		/* NOTE: CCG takes ownership over Subdiv. */
 		result = multires_as_ccg(mmd, ctx, mesh, subdiv);
 		// BKE_subdiv_stats_print(&subdiv->stats);