problems with postscript backend

The problem is in draw_line_collection, and it looks

    > complicated.

I wrote that mess :slight_smile: This is the baseclass method for backends that
do not implement draw line collection. I will fix it for the new
API. You may want to look over the draw_*_collection methods in
backend bases and see if there are any significant performance boosts
to be had by implementing them in backend_ps. If you need more
distraction, that is :slight_smile: