boxpointer: Avoid malformed boxpointer arrow

boxpointer: Avoid malformed boxpointer arrow

If the arrow's origin is so close to the edge that the arrow will not
be isosceles, we try to compensate as follows:
  - We skip the rounded corner and settle for a right angled arrow as
    as shown below.
    |\_____
    |
    |
  - If the arrow was going to be acute angled, we move the position of
    the box to maintain the arrow's accuracy.

https://bugzilla.gnome.org/show_bug.cgi?id=680077