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

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 eglstream-pixmap-release-cb in 9 minutes and 22 seconds (queued for 2 seconds)