I actually did compile that last commit and misread the failed projects
as 0. I'm just going to put the conversion stuff in video-io.h stuff
because it requires it anyway, and video-scaler.h already depends on
video-io.h for the video_format enum anyway.
Add a scaler interface (defaults to swscale), and if a separate output
wants to use a different scale or format than the default output format,
allow a scaler instance to be created automatically for that output,
which will then receive the new scaled output.