For easier creation of rectangular boxes, the module provides the
specialized class rect. Its constructor first takes four
parameters, namely the x, y position and the box width and height.
Additionally, for the definition of the position of the center, two
keyword arguments are available. The parameter relcenter takes
a tuple containing a relative x, y position of the center (they are
relative to the box extent, thus values between 0 and
1 should be used). The parameter abscenter takes a tuple
containing the x and y position of the center. This values are
measured with respect to the lower left corner of the box. By
default, the center of the rectangular box is set to this lower left
corner.
