[cairo] Better Examples of Cairo Operators

[Lawrence D'Oliveiro]

I wrote:

> 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.

Hmm, I tried adding a mask to constrain the compositing to exactly the
area of the source rectangle. But it still wipes out the entire
destination...