Used by the routines at MOVE_MUMMY, MOVE_DRACULA, MOVE_FRANKENSTEIN, MOVE_DEVIL and MOVE_HUMPBACK.

Compares the target's position with its own on each axis and sets the velocity fields to $FF or $01 accordingly -- the crudest possible pursuit, one pixel a frame towards wherever the player is, with no smoothing and no memory. It is what makes some creatures follow you rather than drift.