[cairo] intersect_lines in Bentley Ottman implementation

[M Joonas Pihlaja]

Hi Antoine,

I looked over the patch a bit and can answer your embedded
question about the tests comparing the remainder vs the
denominator after calling the mixed precision divide.  The reason
is that the divide operation uses the rem == den condition to
indicate that the result has overflowed the result's bit width --
so especially there cannot be any intersection that we care
about.  It's a little non-obvious and ugly, and really warrants a
comment explaining it I'll admit.

Cheers,

Joonas