xwayland: Clean up pending eglstream on pixmap destroy

EGLStream implementation in Xwayland keeps a list of pending streams for
a window.

If the windows's pixmap is destroyed while there is a pending stream,
the pending stream will point to freed memory once the callback is
triggered.

Make sure to cancel the pending stream if there's one when the pixmap is
destroyed.

v2:
 * Use xorg_list_for_each_entry() instead of the safe variant (Michel
   Dänzer <mdaenzer@redhat.com>)

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Tested-by: Karol Szuster <karolsz9898@gmail.com>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Closes #1096
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