[Pixman] PDF radial gradients

[Andrea Canciani]

I updated the documentation of radial gradients in my wip/radial branch
(http://cgit.freedesktop.org/~ranma42/pixman/log/?h=wip/radial) to reflect
my changes (which make pixman gradients behave as specified by pdf
reference manual).

The code is not yet ready to be merged (it uses __int128 variables, which
probably won't work on 32 bit architectures), but I would appreciate reviews
(expecially of the new documentation) and testing.

Andrea