The direction vector from the camera through the mouse pointer into
the 3D level, calculated from mouse_pos and
the view parameters. This vector can be used to trace a ray to or from
the mouse position.
while (mouse_mode > 0)
mouse_pos, mouse_mode, mouse_map, mouse_spot, mickey, pointer, mouse_range, mouse_moving, mouse_calm, mouse_time, mouse_left, mouse_ent, mouse_force