[cairo] enhanced Cairo::Context#save

[mental at rydia.net]

Quoting Øyvind Kolås <islewind at gmail.com>:

> I'd prefer if all syntactic sugar was kept in
> rcairo/packages/cairo/lib/cairo.rb
> in the same matter that the syntactic sugar for allowing
> cr.rgb_color=0.2, 0.3, 0.4 should be handled entirely in ruby
> land.

Hmm, I though that's what I had done ... is renaming the
C-implemented method to 'save_internal' a problem?  That's the only
change made on the C side...

-mental