Infinite rendering when provided negative percentage value

[ILW8]

When rendering a beatmap, putting something like -100% as an argument will cause flowabot to infinitely render frames until either it is restarted or /tmp/frames runs out of space.

I think either sanitizing input (only positive values) or implementing reverse playback solves the issue. Obviously, the former's easier than the latter.

[ILW8]

Closed by 2ae69fe