scene/workspacescene: special case video players for direct scanout candidate search

commit 1: scene/workspacescene: extend direct scanout candidate search for multiple planes

commit 2: compositor_wayland: optimize out black background surfaces for direct scanout

If the application uses a surface with a single-pixel black buffer as a background, for example behind a video or video game, that surface can be ignored, as the KMS background is black as well. With this commit, mpv with --vo=dmabuf-wayland can get direct scanout even if the aspect ratio of the video and screen don't match, which can result in very significant power savings