Add coordinate system scaling to custom symbols

Perhaps I did not read the documentation carefully, but I did not find the possibility of scaling the coordinate system in custom symbols code. There is rotation, but no scaling. This would be a useful feature, for example, for drawing arcs of ellipses. It might be worth adding the clip option as well.

Also, file insertion is not well documented. It is not clear how the "x", "y" and "size" parameters of the EPS command relate to the BoundingBox of EPS.