How to 'crop' a g-code

[paulus.v]

A late response on this problem...

In geometry, the term for what you want to do is 'clipping'
Try a search for 'polygon clipping'
It seems like a simple task but is made difficult by lots of tricky edge cases.
If you can code in c++ or c# or Delphi i can recommend Angus Johnsons excellent freeware 'Clipper' http://www.angusj.com/delphi/clipper.php
Cheers

First of all thanks a lot Greeny for pointing me in the right direction!

For a programmer it was a 5 minutes job. The solution was simpler than the clipper library (certainly included in the library as well), it is called ray casting algorithm.
Here are code examples.

I don't think you realise just how complex a mathematical problem what you're asking for constitutes

Nick, I'm sorry to tell you that this complex mathematical problem is done by a computer in less than a second, by comparing 50k points against a 12 points defined polygon.