Add support for animated and non-4:3 backgrounds

[nightmareci]

SDL_image, the library used for loading images, supports loading animated GIFs and WEBPs, so support for animated backgrounds using the animated image support in SDL_image should be added.

In line with improving background graphics functionality, support for non-4:3 backgrounds should also be added. The current game content layout wouldn't be changed, but would be centered on top of the background, regardless of the current background's aspect ratio. Arbitrary aspect ratios should be supported; don't hard code to only support 4:3 and 16:9 backgrounds, for example.

[zaphod77]

this will really mess with the tomoyo eh 18 gimmick., btw. this bit is the real reason why 4:3 is required. that game mode is 100% incompatible with 2 people playing as well, and i don't know how that can be fixed.