Change: decide what axis will determine video quality by screen ratio

The previous method picked the shortest axis, which is safer in wide range
of video aspect ratios (like 4:3). However the old method picked 1080p for
resolutions like 1280:800, where 720p is optimum. The new method based on
screen aspect ratio is more effective for 16:9 videos, but could result in
lower than optimum quality for narrow videos (like 4:3).

Fixes #10.
Change: assume 16:9 video for width based quality determination.

More effective for 16:9, less safe for 4:3. Details at e0aabf0.