Use the scaled video size (i.e. as shown on the window) as reference for
zoom. This is the easiest way to fix different width/height scale
factors as they happen when zooming video with a pixel aspect ratio
other than 1:1.
Also fix the unscaled mode, so that it 1. doesn't scale even with
--video-zoom, and 2. doesn't scale by small amounts when the video is
cropped by making the window smaller than the video.