Decide JPEG support

[historic_bruno]

Currently the game requires libjpeg as a dependency, even though we don't use it for anything. Game art should never use JPEG, but a lossless format like PNG. Really the only thing I could imagine us wanting JPEG for is screenshots, but even on high quality settings it will lose a lot of detail.

On Windows, we use a very old version of libjpeg, but we should decide if it's worth supporting JPEG at all, and if not, just remove it from the game.