xwayland: enable MIT-SHM shared pixmaps

Allow X11 clients to create shared pixmaps via the MIT-SHM
extension under Xwayland. Tested with a wlroots patch [1].

Also add a few assertions to make sure we have wl_buffers where we
need them.

[1]: https://github.com/swaywm/wlroots/pull/2875

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
5 jobs for shm-shared-pixmaps in 4 minutes and 39 seconds (queued for 1 second)
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passed #9861291
debian-buster

00:00:16

 
  Build And Test
passed #9861292
autotools

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passed #9861293
meson

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passed #9861294
meson-noglamor

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passed #9861295
mingw-cross-build

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