Update the fullscreen mode to handle non-4:3 images and non-4:3 screen

areas a lot better. This should overall select the largest image available
(from the list of fixed maximum sizes) that will fit in the client area,
without rounding down to nearest 4:3 size twice. (IOW, it can show a
640x320 image in a 640x320 client area, even though 640x320 isn't large
enough for a generic 640x480 image.)