Improve GUI texture atlasing

There are several things to consider here:

• There has been one report of the atlas generation on the GPU not working properly with mesa driver. Might be a better idea to do it on the cpu.
• There are localizations of MW that use different sizes for some textures (see bug #386 (closed)). This means that we cannot use predefined atlas coordinates and have to calculate them dynamically based on the actual size of the textures (which has the added benefit of not having to specify the coordinates by hand). What has to be specified is the names of the textures and a "context hint", indicating which atlas to put this in (although it is not guaranteed, because we don't know the size beforehand). For example, all the textures of the main menu should have the same context hint because they often appear at the same time.
• Add a save option so it can be loaded from cache on the next run.

(RM-388 from redmine: created on 2012-08-29 by scrawl ., , closed on 2013-01-03 by nobody, relates #386 (closed))