Add alternative ways to draw a rectangle

[fenjalien]

We currently allow rectangles to be drawn by specifying their top left and bottom right coordinates. This technically also allows the top left coordiante and the width and height to be specified as well.

It would be better to allow different ways to specify a rectangle, Processing allows the following:

• CORNER: upper left corner and width/height
• CORNERS: upper left and bottom right (what we do at the moment)
• CENTER: center and width/height

[johannes-wolf]

With 0.4.2 we will have rect-around to draw a rect around a set of coordinates (a bounding box).