xwayland: Add a flag for n-buffers in EGL backend

Using multiple window buffers crashes with EGLStream, which does not
need it anyway as this is handled through EGL directly.

Add a flag to the EGL backend to indicate whether it would benefit from
multiple buffers and use this in the get_buffer() function.

Thanks to Adam Jackson <ajax@redhat.com> for pointing out that issue
with EGLStream.

v2: Fix logical test (Adam Jackson <ajax@redhat.com>)

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
4 jobs for add-egl-backend-flags in 4 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #4636011
debian-testing

00:00:27

 
  Build And Test
passed #4636012
autotools

00:03:49

passed #4636013
meson

00:01:45

passed #4636014
meson-noglamor

00:01:38