dmabuf: Add priorities to formats

This will be used to influence the sorting of formats, so we can prefer 'native' formats over those that we can't import.

Test included.

API in this:

gsize              gdk_dmabuf_formats_next_priority             (GdkDmabufFormats *formats,
                                                                 gsize             idx)

dev_t              gdk_wayland_dmabuf_formats_get_main_device   (GdkDmabufFormats *formats);

dev_t              gdk_wayland_dmabuf_formats_get_target_device (GdkDmabufFormats *formats,
                                                                 gsize             idx);

gboolean           gdk_wayland_dmabuf_formats_is_scanout        (GdkDmabufFormats *formats,
                                                                 gsize             idx);

GdkDmabufFormats * gdk_surface_get_dmabuf_formats               (GdkSurface     *surface);