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xwayland: Remove xwl_present_window from privates on cleanup

Adam Jackson requested to merge ajax/xserver:patchwork-247271 into master 
Xwayland's `xwl_destroy_window()` invokes `xwl_present_cleanup()`
before the common `DestroyWindow()`.

But then `DestroyWindow()` calls `present_destroy_window()` which will
possibly end up in `xwl_present_abort_vblank()` which will try to access
data that was previously freed by `xwl_present_cleanup()`:

  Invalid read of size 8
     at 0x434184: xwl_present_abort_vblank (xwayland-present.c:378)
     by 0x53785B: present_wnmd_abort_vblank (present_wnmd.c:651)
     by 0x53695A: present_free_window_vblank (present_screen.c:87)
     by 0x53695A: present_destroy_window (present_screen.c:152)
     by 0x42A90D: xwl_destroy_window (xwayland.c:653)
     by 0x584298: compDestroyWindow (compwindow.c:613)
     by 0x53CEE3: damageDestroyWindow (damage.c:1570)
     by 0x4F1BB8: DbeDestroyWindow (dbe.c:1326)
     by 0x46F7F6: FreeWindowResources (window.c:1031)
     by 0x472847: DeleteWindow (window.c:1099)
     by 0x46B54C: doFreeResource (resource.c:880)
     by 0x46C706: FreeClientResources (resource.c:1146)
     by 0x446ADE: CloseDownClient (dispatch.c:3473)
   Address 0x182abde0 is 80 bytes inside a block of size 112 free'd
     at 0x4C2FDAC: free (vg_replace_malloc.c:530)
     by 0x42A937: xwl_destroy_window (xwayland.c:647)
     by 0x584298: compDestroyWindow (compwindow.c:613)
     by 0x53CEE3: damageDestroyWindow (damage.c:1570)
     by 0x4F1BB8: DbeDestroyWindow (dbe.c:1326)
     by 0x46F7F6: FreeWindowResources (window.c:1031)
     by 0x472847: DeleteWindow (window.c:1099)
     by 0x46B54C: doFreeResource (resource.c:880)
     by 0x46C706: FreeClientResources (resource.c:1146)
     by 0x446ADE: CloseDownClient (dispatch.c:3473)
     by 0x446DA5: ProcKillClient (dispatch.c:3279)
     by 0x4476AF: Dispatch (dispatch.c:479)
   Block was alloc'd at
     at 0x4C30B06: calloc (vg_replace_malloc.c:711)
     by 0x433F46: xwl_present_window_get_priv (xwayland-present.c:54)
     by 0x434228: xwl_present_get_crtc (xwayland-present.c:302)
     by 0x539728: proc_present_query_capabilities (present_request.c:227)
     by 0x4476AF: Dispatch (dispatch.c:479)
     by 0x44B5B5: dix_main (main.c:276)
     by 0x75F611A: (below main) (libc-start.c:308)

This is because `xwl_present_cleanup()` frees the memory but does not
remove it from the window's privates, and `xwl_present_abort_vblank()`
will still find it and hence try to access that freed memory...

Remove `xwl_present_window` from window's privates on cleanup so that no
other function can find and reuse that data once it's freed.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1616269
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>

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