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>