Used by the routine at WALK_ON.

WALK_ON calls this for every step of the knight's walk; FOOTSTEPS counts them. Walking (speed under 12) every fourth plays four waves, at a half-wave count of 64 and 96 by turns; at the faster speed a bonus gives, every second, so the steps come twice as often. Pentagram's footstep plays two waves, and has no faster half.