Log Message:
update to gerbv-0.13

* You cannot compile with backend anymore, switch is removed. In last
  release this caused compilation error due to bit rot.
* Drawing of arcs is a constant headache. Mark Whitis found another
  case that I had missed out. Clockwise arcs all of a sudden started
  to be drawn as counter-clockwise.
* Arcs with very small angle differences could either be a complete
  circle or a very small part of an arc. Calculating the angles with
  integers wasn't sufficient, anyhow. They are now doubles all the way.
* Aperture macro primitive 1 was a _filled_ circle.
* Image rotate used to cause warnings that it wasn't implemented. Now
  if the file has an image rotate on zero degrees there is no warning
  (yes I've seen it).
* Dan McMahill discovered that if you tried to swap with an unused layer
  gerbv segfaulted.
* Peter Monta submitted patch for incremental coordinates.
* No traces of Guile left...