[cairo] Better Examples of Cairo Operators

[Lawrence D'Oliveiro]

I wrote:

> You can see what the new output looks like here
> <http://default-cube.deviantart.com/art/Operators-520466116>.

For comparison, here
<http://default-cube.deviantart.com/art/Operators-532330493> is an
illustration of the Pixman operators.

For a start, I notice an interesting discrepancy in the behaviour of
the “clear” operators: the Pixman versions seem to wipe out the entire
destination, even where the source is fully transparent.