[cairo] Antialiasing renderer

[mental at rydia.net]

Quoting Jouni Tulkki <jouni.tulkki at hut.fi>:

> The renderer renders polygons only. Code to convert lines and
> other shapes into polygons is yet to be written.

In the context of a hypothetical cairo backend based on this, that
wouldn't be needed -- cairo takes care of converting everything
into trapezoids for you.

> Texturing and other advanced features are also not available.

You might be able to take advantage of libpixman and other cairoish
facilities to accomplish that, rather than writing it yourself.

-mental