Animation support

A made-up signal serving as a crude animation testbed.

This morning I added animations and playback state. Animation data exists once, and each playback state - i.e. each sprite that is actually playing that set of frames - is just a 'play head' that references the animation data. The state can loop and playback at a different framerate independent of the original animation.

None of this stuff was terribly hard to add, but it will become important in making the world feel alive.

(Creating a test with aseprite also made me realise that an easy enhancement for the future will be to make frame times per frame, rather than a fixed FPS for the entire animation)