Add a new DisplayScanout structure to save the current scanout details.
This allows to attach later UI backends and set the scanout.
Introduce displaychangelistener_display_console() helper function to
handle the dpy_gfx_switch/gl_scanout() & dpy_gfx_update() calls.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>