GameSettings and Options GUI overhaul.

* Overhauled GameSettings. Use Settings enum instead of integer literals to work with settings.
* Made the settings file parser more permissive: key names are now case-insensitive, "true" literal is also case-insensitive, and "1" is accepted in its place.
* Took out 3D Anaglyph functionality.
* Updated [Options] and [Advanced Options] screens -- some settings have been rearranged.
* Changed [Anisotropic] setting to allow different anosotropy levels, instead of just being on/off.