Used by the routine at SPIKES.

Graphics 63 and 79 (blocks), 72 (a tree stump), and the tails of many other routines. A push from something walking into it (the collision code copies the pusher's step into a pushable thing's) moves it that far this turn; it also falls one unit a turn, since the fall at DEC_DZ_AND_UPDATE_UVZ takes one off the Z step and STOP_MOVING clears it again.

STOP_MOVING is the common ending: if it moved at all this turn, clear the three steps and have it redrawn; if not, return without a redraw, so a thing at rest costs nothing to draw.