xwayland: avoid using freed xwl_window on unrealize

xwl_unrealize_window() would use freed xwl_window which can lead to
various memory corruption and crashes, as reported by valgrind:

 Invalid read of size 8
    at 0x42C802: xwl_present_cleanup (xwayland-present.c:84)
    by 0x42BA67: xwl_unrealize_window (xwayland.c:601)
    by 0x541EE9: compUnrealizeWindow (compwindow.c:285)
    by 0x57E1FA: UnrealizeTree (window.c:2816)
    by 0x581189: UnmapWindow (window.c:2874)
    by 0x54EB26: ProcUnmapWindow (dispatch.c:879)
    by 0x554B7D: Dispatch (dispatch.c:479)
    by 0x558BE5: dix_main (main.c:276)
    by 0x7C4B1BA: (below main) (libc-start.c:308)
  Address 0xf520f60 is 96 bytes inside a block of size 184 free'd
    at 0x4C2EDAC: free (vg_replace_malloc.c:530)
    by 0x42B9FB: xwl_unrealize_window (xwayland.c:624)
    by 0x541EE9: compUnrealizeWindow (compwindow.c:285)
    by 0x57E1FA: UnrealizeTree (window.c:2816)
    by 0x581189: UnmapWindow (window.c:2874)
    by 0x54EB26: ProcUnmapWindow (dispatch.c:879)
    by 0x554B7D: Dispatch (dispatch.c:479)
    by 0x558BE5: dix_main (main.c:276)
    by 0x7C4B1BA: (below main) (libc-start.c:308)
  Block was alloc'd at
    at 0x4C2FB06: calloc (vg_replace_malloc.c:711)
    by 0x42B307: xwl_realize_window (xwayland.c:488)
    by 0x541E59: compRealizeWindow (compwindow.c:268)
    by 0x57DA40: RealizeTree (window.c:2617)
    by 0x580B28: MapWindow (window.c:2694)
    by 0x54EA2A: ProcMapWindow (dispatch.c:845)
    by 0x554B7D: Dispatch (dispatch.c:479)
    by 0x558BE5: dix_main (main.c:276)
    by 0x7C4B1BA: (below main) (libc-start.c:308)

This is because UnrealizeTree() traverses the tree from top to bottom,
which invalidates the assumption that if the Window doesn't feature an
xwl_window on its own, it's the xwl_window of its first ancestor with
one.

This reverts commit 82df2ce3

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>