Add scrolling animation for moving between screens

[potatolain]

Games like Legend of Zelda do a pretty scroll animation when you transition between screens... it shouldn't be that hard to implement here.

(Note: This is actually about halfway implemented - source is in map.c - for now we just have a clean looking fade animation instead.

[potatolain]

Okay "shouldn't be that hard" may have been the wrong description... it took way more effort than I'd like to disclose.

That said, this is finally done! The game feels significantly more polished now. Fixed with PR #6