* render,callbacks,interface: Restructure the whole rendering process by
separating out all zoom and pan information from the global variable
"screen". This was necessary to allow rendering without having a gtk
window open, allowing for export from command line and many others.
Also, the translation and zoom code is much more readable due to the
elimination of many variables. Also, the GDK color allocations were
eliminated to allow rendering without existing GDK colormaps in place.
* gerbv.c: Create the basic framework for exporting from the command line.