Have Tux swim instead of floating when going up/down

[n0toose]

This is a small detail that I mostly made so as to get used to the codebase.

If the vertical velocity is under a specific threshold, then the floating animation will be used. Otherwise, if v is too low but vy is too high, show the swimming animation instead.

I feel like the change in itself makes sense, even if it won't be noticed by any player that isn't willing to move around in squares.

[n0toose]

(Maybe the commit message is a bit misleading.)