[cairo] Cairo backend: Implementing composite

[Christian Biesinger]

Hi,
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?

And, how does it interact with the matrix? Is the implementation 
supposed to take a rectangle at (src_x, src_y) with dimensions (width, 
height) from the source pattern, then transform it according to the 
matrix, then put it into (dst_x, dst_y) clipped to an area of (width, 
height)?

thanks,
-biesi