[cairo] Painting an image surface into itself in order to scroll it

[Nicholas Allen]

>
>> It seems cairo has a bug that this is not possible with just one image
>> surface if you scroll the contents downwards. I can work around it by
>> creating 2 image surfaces and copying one to the other but I would
>> expect cairo to be able to handle this with just one surface.
>
> If you don't do the copy to an extra surface, cairo would have to do 
> it for you. This means that the resulting performance is equally bad. 
> Yes, there could be shortcuts for the image backend, but I think most 
> other (raster) backends would have to do the extra copy.

I don't see why cairo would need to do the copy - it can just iterate 
over the pixels in a different order (right to left instead of left to 
right for example). That way it doesn't overwrite pixels as it does the 
blit. I just wanted to save memory here but if cairo doesn't support 
this then I'll just change the code to use 2 image surfaces but this 
seems rather wasteful of memory...

Cheers,

Nick