MTDisplay: SDL backend

[AmEv7Fam]

Flib and I got talking, and we think that this is currently the best way to go.

[flibitijibibo]

Yup, just an MTSDLDevice and an MTSDLBitmap. Can be done with SDL and SDL_image, respectively.

[flibitijibibo]

Oh, and MTDecode can be shorted to a handful of SDL_image calls. Yeah.

EDIT: Nope, this was just a bunch of junk for MTGDIBitmap.

[flibitijibibo]

Okay, so we're all set to go here. Basically just implement MTBitmap, MTMask and MTDisplayDevice for SDL. They can be implemented in MTSDLDevice.cpp for MTDisplayDevice and MTSDLBitmap.cpp for MTBitmap and MTMask. Soon as that's done, this can be closed as "finished", as debugging it is a whole different story.