Basic image loading

Created by: IanS5

Added basic image loading. Unless SDL/SDL_image.h has been included it can only load bitmaps. If the image does not fit the screen perfectly the config option allow-wallpaper-margins = <bool> can be used; if true it will force the entire image into the frame (but there will be black margins), if false some parts of the image may fall off the screen but it will cover the entire thing. screenshot