[cairo] Disabling antialiased shape drawing

[Billy Biggs]

  Attached is a second attempt at the patch which addresses Owen's
comments and uses cairo_antialias_t.  I moved it from the font docs up
to the cairo_t docs and changed the wording around in the antialias type
to make it more generic.

  A comment though:

Carl Worth (cworth at redhat.com):

> > Con for reuse:
> > 
> >  - Not clear how meaningful SUBPIXEL is for this. Subpixel geometry
> >    has some definite challenges.
> 
> Ah, but I'd put this one in the Pro list. It'd be odd now, since we
> don't have subpixel geometry yet. But subpixel geometry is definitely
> something I've wanted cairo to be able to do eventually. [...]

  I am a bit worried about defining an API before we can prove it
sufficient.  Presumably if you want to implement subpixel geometry,
you'd need to know its orientation, so you'd have to set that somewhere,
which may make this separate setting redundant.  However, if this
is just a hint then it might not matter so much (make default be
something defined by context).

  This patch also explicitly says that the setting is a hint which may
not be implemented by the backend.  I think this is correct but maybe
roc has a different opinion here.

  -Billy