x11: Fix damage tracking hack

x11: Fix damage tracking hack

We are setting mime data with a destroy notify on the cairo
surface to get notified when cairo registers damage for the
surface (in that case, it clears the mime data, calling the
destroy notify). Unfortunately, the destroy notify is also
called when we remove the mime data ourselves, which was
not intentional.

Use a flag in the window impl struct to ignore the callback
when we are clearing the hook.