[cairo] composites circle

[Lawrence D'Oliveiro]

On Sat, 16 Jan 2016 23:55:52 +0100, Alex Vazquez wrote:

> Code:
> http://pasted.co/634b16c6

This does not seem to be using alpha transparency anywhere.

But going back to the original example
<http://postimg.org/image/ieohdu29t/>, I think I previously
misunderstood what was going on. Each disc is opaque outside the
intersecting area. The intersecting part has a mix of the two colours.

So really, draw each circle with an opaque colour clipped outside the
other circle. Then fill the mixed colour, clipped to the intersection
of the two circles.