xwayland: Don't create wl_buffer backing pixmap

In non-rootless mode, not all pixmaps need a wl_buffer backing.

Suggested-by: Twaik Yont (@twaik) in #834
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
3 jobs for xwayland-rootless-backing in 8 minutes and 17 seconds (queued for 1 second)
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autotools-build-and-test

00:06:27

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meson-build-and-test

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