The shift from margins should be applied only to the line's position,
not to each point in the drawing, and the scale needs to compensate for
that the scale on the style is being changed as well.
Add modes where the borders to add or remove are automatically
calculated rather than forcing the user to do it manually, and hopefully
make it a bit clearer what exactly will happen.
ptr_vector hasn't been updated for C++11, so despite being specifically
designed to store pointers to objects it's less safe and not really any
easier to use than a regular vector of unique_ptrs
