xwayland: Do not keep the cursor's pixmap aroundHEADmaster

Currently, Xwayland creates a pixmap backed by shared memory buffer as
soon as an X11 cursor is realized, which is destroyed when the cursor is
eventually unrealized.

If an X11 client is leaking cursors, Xwayland will be creating new
pixmaps continuously, which will eventually cause an error once the
limit is reached, and get Xwayland killed.

However, we do not need the shared memory buffer to stay around, we
already have the buffer retention mechanism which will take care of
keeping the buffer around until the Wayland compositor is done with it,
so we could just create and destroy the pixmap as needed when setting
the cursor.

That would not fix the leak in the X11 application, yet that would
mitigate the risk of Xwayland being killed by reaching the shared memory
limits, until the client itself reaches the limit of X11 resources.

v2: Don't increase the pixmap refcnt to destroy it just after (Michel)

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Suggested-by: Michel Dänzer <michel@daenzer.net>
See-also: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1773
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1754>

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