SDL: RFC - Add option to render GUI in 32-bits

As some of you may know, I've been working on implementing 32-bit rendering of the GUI on the SDL-backend as part of GSoC 2013. Basically, the functionality is implemented, however - I would like to have some comments before proceeding further, if this way of implementing it will break stuff, suggestions on improvements etc. etc. 

Basically there is one known issue, and that is that changing the theme will break stuff. This has to do with it checking the mode-flag, which I would suggest breaking up into two parts (one for bit-depth and one for standard/aa -rendering).