Xwayland's own SetWindowPixmap() handler would be ignored when running
rootful.

This is fine as long as we do not plan to resize the root window,
however this is becoming problematic if we plan to resize the root
window dynamically when running rootful.

Just add the xwl_window_set_window_pixmap() handler regardless of
rootful/rootless mode.

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