constrainttree: Fix criticals

We don't want the edit constraints to be required,
since we want to violate them when the pointer moves
off the edge. Make them strong instead, and now
everything just works.

tests/constrainttree.c

@@ -214,10 +214,10 @@ drag_begin (GtkGestureDrag *drag,
 
   gtk_constraint_solver_add_edit_variable (solver,
                                            drag_node->x_var,
-                                           GTK_CONSTRAINT_WEIGHT_REQUIRED);
+                                           GTK_CONSTRAINT_WEIGHT_STRONG);
   gtk_constraint_solver_add_edit_variable (solver,
                                            drag_node->y_var,
-                                           GTK_CONSTRAINT_WEIGHT_REQUIRED);
+                                           GTK_CONSTRAINT_WEIGHT_STRONG);
   gtk_constraint_solver_begin_edit (solver);
 }