[cairo] composites circle

[Lawrence D'Oliveiro]

On Wed, 13 Jan 2016 10:52:37 +0100, Alex Vazquez wrote:

> I am trying replicate the next effect with cairo
> http://postimg.org/image/ieohdu29t/

That looks like 50% transparency between the circles, but opaque with
respect to the background.

This has to be done in two stages, rather like
<http://default-cube.deviantart.com/art/Figure-Alpha-548702174>.