[cairo] Dithering in master

[Behdad Esfahbod]

Hey,

I finally cleaned up the dithering branch, fixed
acquire_surface_for_solid enough, and pushed it all.

The new dithering looks much better than my previous attempt.  The
previous report is here:

  http://www.nabble.com/Dithering-td16411729.html

Previously color-smear looked like this:

  http://imagebin.ca.tbcdn.com/img/nGPfqYz.png

It now looks like this:

  http://imagebin.ca.tbcdn.com/img/cZ8S1Ky.png

Top-right counter-clockwise is dithered color-smear for cubes of size 6,
5, 4, 3, and 2.  6 being the default.

I also fixed 16-bit dithering:

  http://imagebin.ca.tbcdn.com/img/PxAGqYP.png

Left to right: 32bit, 16bit dithering, 16bit no dithering.

The only catch is that dithering only works if render is not available.
But if someone is running *with* render and on 8bit, that's their bug
IMO.


Comments and rotten eggs welcome.

-- 
behdad
http://behdad.org/

"Those who would give up Essential Liberty to purchase a little
 Temporary Safety, deserve neither Liberty nor Safety."
        -- Benjamin Franklin, 1759