Used by the routine at player_move_and_draw.

Works out this frame's move, has the collision code cut it short where it must, lets the player leave the room if that is what the move does, and applies it.

While an object is floating up into the wizard's cauldron ($5BC4 set) the controls are off and dZ is forced to 2 here, which the two units of gravity below bring back to 0: the player hangs where he is.