This command now supports option for rounded corner using the following syntax:
//RECTANGLE, left, top, right, bottom {,fill {,crwidth {,crheight}}}
where crwidth and crheight refer to the "corner radius" width and height, in the current mapmode units. For normal (symmetrical) rounded corners, crheight is not needed, and crwidth should be set to the radius of the circle used to draw the arc of the rounded corner. When crheight is specified, the corners are made from the arc of an ellipse whose total height (bounding rectangle) is two times crheight and whose total width is two times crwidth.
