[cairo] Cairo backend: Implementing composite

[Christian Biesinger]

Christian Biesinger wrote:
> I've got a question about the width/height argument to the backends' 
> composite functions. Does it refer to the destination rect only, or also 
> the source and mask rects?

OK, I thought about this a bit... I now believe that what makes most
sense is:
- transform the surface according to the matrix
- Take the specified rectangle of it (using x/y and width/height)
- composite that onto the dest surface at x/y and width/height

This means that EXTEND_REPEAT and such would only have an effect if the 
surface is smaller than the rectangle or gets transformed to be smaller.

Is that the right behaviour?

-biesi