[cairo] enhanced Cairo::Context#save

[mental at rydia.net]

Quoting Øyvind Kolås <islewind at gmail.com>:

> this is what I commited to CVS:
>
> +    alias save_internal save

Oh.  Yes, that is a lot better.  We may as well also make
save_internal private too.

> If this were to change in the .c file, I'd prefer doing the yield
> there as well. It's a matter of taste though.

There are some practical considerations too; for example, you can't
(usefully) take a continuation from within the block if there are C
stack frames involved.

(that is also why for current_path, I return an array from C and
iterate over it in Ruby)

-mental