[Pixman] Dithering patches, v2

[basile-pixman at clement.pm]

I am resubmitting for review and inclusion my series of patches on
dithering from October, which I ended up not having the time to finish
then.  There are only a couple changes to make the patches compatible
with the recently added floating point formats, and I have also added a
dithering matrix based on blue noise which provides more pleasant
results than the Bayer matrix from the first series.

Maarten, I would appreciate if you can take a second look (although as
noted above not much has changed since your first review -- I don't
think you had comments on the dithering part?).

- Basile