[cairo] Disabling antialiased shape drawing

[Billy Biggs]

Robert O'Callahan (rocallahan at novell.com):

> >  2. The API can be safely ignored by backends that do not require this
> >     setting, such as the PS or PDF outputs.
> 
> Making this API non-mandatory means that an application that wants to
> use supersampling has to know, for each surface type it uses, whether
> or not the surface type really supports this API. But such
> applications already need to know which surface types need
> supersampling, so I guess this is OK, as long as a surface type's
> support for this API is considered part of the interface and isn't
> removed in the future.

  I think you need something else.  I want to ask a backend to disable
antialiasing, and I think you want to ask if you should do supersampling
for it.  I don't think that one pair of functions should try to do both.

  -Billy