Long version:

UBGraphicsPolygonItem::shape() sometimes caused OpenBoard to crash due to
inifinite recursion. This is easily replicated by trying to use the
highlighting tool.

The reason is: shape() calls boundingRect(); this function's definition is:

     QRectF QGraphicsPolygonItem::boundingRect() const
     {
          Q_D(const QGraphicsPolygonItem);
          if (d->boundingRect.isNull()) {
              qreal pw = pen().style() == Qt::NoPen ? qreal(0) : pen().widthF();
              if (pw == 0.0)
                  d->boundingRect = d->polygon.boundingRect();
              else
                  d->boundingRect = shape().controlPointRect();
          }
          return d->boundingRect;
      }

In the case where pw != 0, the shape() function is called. However, it
is shape() from the derived class, not the base class, which is called.
In other words, UBGraphicsPolygonItem::shape() is called rather than
QGraphicsPolygonItem::shape().

This means that boundingRect() is called again from within shape(), and
so on, causing the program to crash.

The fix was simply to remove UBGraphicsPolygonItem::shape(), as it
appears to provide the same (or very similar) functionality to that of
the base class's shape() function.

In case this shape() function actually is needed, another workaround
should be implemented to prevent this infinite recursion.