Used by the routines at dec_dZ_wipe_and_draw, upd_182_183, upd_91, guard_ew_move, spiked_ball_drop, fire_ew_move, fire_ns_move, ud_ball_rise_or_fall, ball_up, upd_160_to_163, chaser_chase, upd_30_31_158_159, move_charm_to_cauldron, upd_96_to_102, upd_85, dec_dZ_upd_XYZ_wipe_if_moving, upd_80_to_83, upd_9 and move_portcullis_up.

Takes one off dz, then lets adj_for_out_of_bounds cut the velocity down to what the room's walls and floor and the other objects allow (and set the blocked bits in +$0C), and moves the object by what is left.