[Pixman] Gradients patches

[Andrea Canciani]

This morning I pushed
http://cgit.freedesktop.org/~ranma42/pixman/log/?h=radial-for-master
It should be ready for master since it is documented, tested (at least
on my laptop) and not using
any new features (so it should not be broken on other
architectures/compilers/etc).

I'm reviewing the fix for the precision of linear gradients, but I
noticed that there are some
(apparently correct) assumptions that I didn't document. I'm checking
that they are true and
adding explanations of why they hold.

Andrea