create a smaller, scaled window if the screen size doesn't support the default dimensions.

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Running xpost in VirtualBox Ubuntu
2. 612x792 window doesn't fit in 800x600
3.

What is the expected output? What do you see instead?

If the screen doesn't support the requested dimensions, xpost should
create a smaller window of the correct proportions. We can use the
device's default matrix to compensate, so the ps program still sees
612x792.

May need to adjust fillpoly to ensure that narrow lines are still 
visible. 0-width will still bypass to a single-pixel line. But lines
of width 1 may disappear with the current polygon fill algo.

Please use labels and text to provide additional information.

Original issue reported on code.google.com by luser.droog on 17 Dec 2013 at 7:50