vo_wayland: pixel perfect buffers

1.  Separate buffer and temporary file handling from the vo to make maintenance
    and reading code easier

2.  Skip resizing as much as possible if back buffer is still busy.

3.  Detach and mark osd buffers for deletion if we want to redraw them and they
    are still busy. This could be a possible case for the video buffers as
    well. Maybe better than double buffering.

All the above steps made it possible to have resizing without any artifacts
even for subtitles. Also fixes dozen of bugs only I knew, like broken subtitles
for rgb565 buffers. I can now sleep at night again.
wayland_common: fix changing videos when fullscreen

I broke it again.