When using vblank (either GLX or Xpresent), a previous paint request may
still be pending while new damage notifications are triggered by clients
updates.

If that occurs, xfwm4 does not cancel the existing timeout previously
created timeout (so it will retry automatically) but clears the timeout
id.

As a result, if new damage occurs, a new timeout would be created while
an existing one is still running.

When using Xpresent while the screen is turned off, no Xpresent event
notification occur, which increases the risk of the above occuring.

Clear the timeout id only when the repaint succeeded, so we don't pile
up multiple retries.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
See-also: #502