refactor shapes, use mid_y for a corner

Reverse engineering and debugging reveal that:
- AF is corners array of a bounding box:
             AF[3]───────AF[2]
                │           │
 mid_y(&AF[1])→ │  midpoint │
                │           │
             AF[0]───────AF[1]
                      ↑
                  mid_x(AF)
- AF[3] is top left corner of the bounding box
- AF[3].y == AF[2].y
- AF[0] is bottom left corner of the bounding box
- AF[0].y == AF[1].y

Refactoring:
   AF[2].y + (AF[0].y - AF[3].y)/2
-> AF[2].y + (AF[1].y - AF[2].y)/2
-> AF[2].y/2 + AF[1].y/2
-> (AF[2].y + AF[1].y)/2
-> mid_y(&AF[1])