gtk-demo/dnd: Fix up drag_cancel

This needs to return a boolean, also it should not call end because
that will be called anyway by Gtk+ after cancel, and this was causing
warnings due to the opacity being unset with no dragged widget set.

demos/gtk-demo/dnd.c

@@ -53,12 +53,12 @@ drag_end (GtkDragSource *source,
   gtk_widget_set_opacity (item, 1.0);
 }
 
-static void
+static gboolean
 drag_cancel (GtkDragSource       *source,
              GdkDrag             *drag,
              GdkDragCancelReason  reason)
 {
-  drag_end (source, drag);
+  return FALSE;
 }
 
 typedef struct {