[cairo] Optimize spans in the trapezoid rasterizer

[Billy Biggs]

Carl Worth (cworth at cworth.org):

> But, the patch doesn't seem to give correct results for me. Here's
> what I see for the fill-rule test for example:
> 
> http://cairographics.org/~cworth/images/fill-rule-image-out-broken.png
> 
> (Where the star shapes are intended to be filled, of course.)

  While this does look like something I could break with my patch, I
cannot reproduce this problem here.  Can anyone else?

  -Billy