Xwayland will add and remove CRTCs as Wayland outputs are added or
removed.

If there is a pending flip when this occurs, the
`xwl_present_sync_callback()` will be triggered after the Xwayland
output's RRCtrcPtr has been destroyed, hence causing a crash in Xwayland
while trying to use freed memory:

  #1  abort ()
  #2  OsAbort () at utils.c:1350
  #3  AbortServer () at log.c:877
  #4  FatalError () at log.c:1015
  #5  OsSigHandler () at osinit.c:156
  #6  <signal handler called>
  #7  dixGetPrivate () at ../include/privates.h:122
  #8  dixLookupPrivate () at ../include/privates.h:166
  #9  present_screen_priv () at present_priv.h:198
  #10 present_wnmd_flip () at present_wnmd.c:358
  #11 present_wnmd_execute () at present_wnmd.c:466
  #12 present_wnmd_re_execute () at present_wnmd.c:80
  #13 xwl_present_sync_callback () at xwayland-present.c:287
  #14 ffi_call_unix64 () from /lib64/libffi.so.6
  #15 ffi_call () from /lib64/libffi.so.6
  #16 wl_closure_invoke () at src/connection.c:1006
  #17 dispatch_event () at src/wayland-client.c:1427
  #18 dispatch_queue () at src/wayland-client.c:1573
  #19 wl_display_dispatch_queue_pending () at src/wayland-client.c:1815
  #20 wl_display_dispatch_pending () at src/wayland-client.c:1878
  #21 xwl_read_events () at xwayland.c:814
  #22 ospoll_wait () at ospoll.c:651
  #23 WaitForSomething () at WaitFor.c:208
  #24 Dispatch () at ../include/list.h:220
  #25 dix_main () at main.c:276

To avoid the issue, get the `ScreenPtr` from the window instead of the
CRTC that might have been just freed, `xwl_present_flip()` has no use
for the CRTC anyway.

Bugzilla: https://bugs.freedesktop.org/108249


Suggested-by: Michel Daenzer <michel.daenzer@amd.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Daenzer <michel.daenzer@amd.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit b768b7d6)