A 2D floating-point box class. This class has methods and
operators to clear the box, and update its size (bounding
rectangle) by specifying additional points. All methods of this
class are implemented by the system.
Data Members:
BOOL empty;
Indicates if the box is empty. When the += operator is used to
update the size of the box, if the box is empty, the box corners
are set to the point.