Added _box_round_corners to Generator, which draws a box with a given rounding radius for each corner. This is a generalization of what was already done with Twopad and smaller Dilpad components. (Evidently the arc drawing in Twopad was screwed up in the initial translation; this fixes that.) Note that this slightly changes the order and direction of the drawn lines/arcs to consistently flow clockwise starting with the upper-left corner, which is why the SOT363 footprint changed.