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).

Some comments: Since the games/scalers don't play nice with 32-bit rendering, what I've done as a workaround is to create a setScreenFormat-function, which reinitialize the various surfaces(_hwscreen, _tmpscreen etc) with the format specified on-the-fly. This is called from the showOverlay (to switch to 32-bits) and hideOverlay (to switch back to 16-bits)-functions. the _preferredFormat-variable carries the preferred format used when in-game and the _preferredOverlayFormat has the format for the overlay. 

The clearOverlay-function has also been changed - since changing the format messes up the gamescreen, we simply copy over what's currently being rendered (after we hide the cursor) to the variable _overlaybackground, which is later (after conversion) blitted unto the overlay, before adding the widgets and such.

Most of the other changes are just to "un-hardcode" various constants. I've also added an feature for the backends that support 32-bit GUI-rendering.

If anyone wants to see the commits which have been cherry-picked they can be found at https://github.com/rundfunk47/scummvm/tree/32bitguioverlay2 .