Used by the routines at clip_dZ_to_floor, dX_obj_shorten, dY_obj_shorten, dZ_obj_shorten, clip_dX_test and clip_dY_test.

Moves A one step towards zero -- down if positive, up if negative -- and leaves Z set if nothing is left. Every loop in the collision code uses it to cut a move short one unit at a time and try again.