Used by the routines at dec_dZ_and_update_XYZ and move_player_apply.

Every moving object's move comes through here, the player's from move_player_apply and everyone else's from dec_dZ_and_update_XYZ. It takes dX, dY and dZ from the record and shortens each where the object would otherwise end up inside the floor, a wall, or another object, then writes them back.

The axes are done one at a time, Z first, then X, then Y, and each test uses the moves already accepted on the earlier axes (the later ones count as zero). That is what lets a blocked move slide: walking diagonally into a wall keeps the part of the move that runs along it.

While it works, bit 1 of the object's +$07 is set, which is what makes the object scans (is_object_not_ignored) pass over the object itself; found already set, it means the object is not to be moved this way at all. Bits 0-2 of +$0C are cleared at the start and set for each axis whose move was cut.