libweston: Add the ability to determine if a dmabuf is scanout-capable

libweston: Add the ability to determine if a dmabuf is scanout-capable

Adds a new callback 'can_scanout_dmabuf' in weston_backend, which
can be set by the back-end do determine if the buffer supplied can be
imported directly by KMS.

This patch adds a wrapper over it, 'weston_compositor_dmabuf_can_scanout'
which is called before importing the dmabuf in the GPU if the
direct_display dmabuf is being set. If that's true and the check
failed, we refuse to create a wl_buffer.

This patch avoids importing in the GPU.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>