Improve arc pics

[jsorva]

Creating arcs is based on defining an ellipse and the arc in terms of that. Currently, it seems that when you create an arc, the size of the resulting picture is the size of the whole ellipse rather than just the arc, which is not ideal.

Also, arcs have a rotationAngle parameter, which seems to work in a surprising way. I didn’t investigate properly, but presumably this is related: the entire ellipse pic gets rotated, rather than just the arc?