Changed what the bbox_* attributes of StellarClass mean.

The meaning of bbox_x and bbox_y has been clarified; this has not
changed.

bbox_left, bbox_right, bbox_top, and bbox_bottom now refer to the
same thing that they refer to in Game Maker: the location of the
bounding box in the room. I don't know what I was thinking when
I made them what they were previously.