xwayland: Use the pixmap release callback with EGLStream

Commit 77658741 - "xwayland: Add buffer release callback" added an API
to deal with Wayland buffer release callbacks.

The EGLstream implementation has its own wl_buffer callback, move that
to the buffer release API instead so we don't have to deal with Wayland
buffers directly and match the other Xwayland pixmap backend
implementations.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Tested-by: Erik Kurzinger <ekurzinger@nvidia.com>
4 jobs for master in 7 minutes and 14 seconds (queued for 3 seconds)
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passed debian-buster #5694534

00:00:13

 
  Build And Test
passed autotools #5694538

00:07:01

passed meson #5694541

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passed meson-noglamor #5694544

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