With the drm output now waiting for the page flip on render *and* gbm and dumb buffer displays supporting triple buffering,
we don't need the entire async page flip handling logic anymore.
In the future we could turn wait_for_page_flip() into an async fn.
This assumes the same xrgb888 interface that the drm dumb buffer uses, too. Missing, beyond different pixel depths is:
- mode setting (requires fbset on the command line right now)
- vsync (not sure if possible)
- line padding
