Map and unit highlighting

I'm slowly going through all the map rendering code to make sure that all the region / unit highlighting works with no graphics artefacts.

Firstly, if you see something that looks like a bug in the gfx rendering, then let me know here.

Secondly, if you actually think the way it is done right now is wrong in some other way (perhaps the highlight is too dark, or something), then let me know that as well - all comments welcome!