There was a lot of code all for the purpose of parsing arguments when
there's plenty of libraries that do just that. Clap's a bit hefty, but I
chose it anyway.

Important change in command line syntax: The confusing and complicated
10x10, 10x, x10, syntax is now gone, and replaced with --width and
--height flags. If only one is set, the other is inferred from the image
dimensions using the aspect ratio.