[cairo] Seams with image backend again

[M Joonas Pihlaja]

Hi Soeren,

On Thu, 24 Jun 2010, Soeren Sandmann wrote:

> The program below shows that the image backend can produce seams
> between abutting polygons,

The seam occurs because the two abutting edges are rasterised using 
different methods on the scanlines where the seams show.  Turning off 
analytic coverage computation entirely fixes this, but obviously also 
slows things down.  The problem is that when drawing quads 
individually the rasteriser only gets a partial picture of the 
geometry at once, so its heuristics for choosing between analytic 
coverage vs. supersampling end up fighting each other in the big 
picture.

If we could get the two methods to yield bit-exact identical results 
and only differ in speed then that would solve it, but I'm not sure if 
that's easily doable.

Cheers,

Joonas