app: Fix on canvas widgets ignoring clicks from tablets - bug 614441

Disable extended input events when the cursor moves to a child of
the canvas widget. Otherwise GTK will try and fail to deliver an
extended event to the child widget, and end up sending it to the
canvas instead.

app/display/gimpdisplayshell.h

@@ -174,6 +174,8 @@ struct _GimpDisplayShell
   gboolean           keyboard_grabbed;
   guint32            keyboard_grab_time;
 
+  gboolean           inferior_ignore_mode;
+
   gboolean           space_pressed;
   gboolean           space_release_pending;
   const gchar       *space_shaded_tool;