Arbint subtraction/addition deal with borrow/carry correctly, maybe

Eventually they will be right.

This still doesn't fix the wierd bezier bug or the growing digits really fast (exponential?). I suspect the former is related
to pow(Rational<Arbint>, Rational<Arbint>) and the latter probably makes sense if you do the theory (considering how fast Rational<int64_t> overflowed).
Clear the carry/borrow flag before add/sub
Merge branch 'master' of git.ucc.asn.au:/ipdf/code

We both fixed bugs.
Fix GPU rendering a bit. Will probably break >1 ob
Divide some numbers by 5.
C++11 lambdas - are they really worth it?
Zooming is now sligtly less slow.
Merge branch 'master' of git.ucc.asn.au:ipdf/code
Now with GNU MultiPrecision (GMP) integers

gmpint.h is (in theory) mostly inline and shouldn't add much overhead
and was simpler than trying to understand the C++ "binding".

The Str() function is kind of terrible but that can't be helped.

Also fixed bug in realops tester (even the testers have bugs...)
And removed asm tester because the asm function is gone.

The bezier testers now don't compile, I don't think we need them anymore.