[cairo] turn off antialiasing

[Peter Dennis Bartok]

>Simply saying "it's part of the API" doesn't really give enough
>information. For one thing, without an understanding of *why* someone
>using your API is going to anti-aliasing off, we have no idea of
>what rasterization rule is appropriate.
>
>Is the concern speed? Is it OK if a backend that can render antialiased
> polygons as fast as non-antialiased polygons antialiases everything?
>
>Is the concern crisp edges on horizontal/vertical geometry?
>
>or ...?

I was hoping that Carl or Keith would jump in at some point, since we (the 
Mono guys using Cairo to implement GDI+ on X11) went through this discussion 
already 6 months ago, providing several arguments why it should be able to 
turn of AA in Cairo. You might want to look through the archives. I seem to 
remember that Carl and Keith (?) came to some conclusions and commited to 
provide some sort of code-compromise for this. Seems to have been forgotten 
by now, since I am too busy to constantly nag...

Here's a few points why we needed it:
- Because GDI+ provides such a way.
- When drawing GUI controls it is desirable to turn off AA to get crisp 
edges, not only on horizontal/vertical, even for things like circles, if 
they're small enough, to avoid 'smudging'.
- When drawing text with small fonts, to actually be able to still read the 
text.
- Because the default in GDI+ is off, and apps that run on Win32 will not 
have the same behaviour (results) when running on Mono and our GDI+ lib 
based on Cairo.

Cheers,
  Peter