[cairo] Better Examples of Cairo Operators

[Lawrence D'Oliveiro]

I wrote:

> 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.

OK, thinking about it, the “discrepancy” is not due to any difference
in the meaning of “clear”, but is down to the difference in what I am
actually compositing: in the Cairo example this is just one patterned
rectangle, where in the Pixman one it is the entire source Image
containing said rectangle.

Now to decide which one I should change to be consistent with the other
one... :)