If somebody really wants to get in there and get development started on this again, that would be great! Maybe Cairo can be saved. There was hope once upon a time that all the toolkits would use Cairo for drawing. Instead all the toolkits are writing their own rendering code, a redundant waste of resources. Imagine if all that work had gone into Cairo!
The xlib backend is partially implemented by pixman and that has been a problem as there is even less maintenance of that. If I understand correctly, pixman is supposed to be in the X server, but such remote rendering is used very little, or none, by Cairo (the xlib backend triggers a switch to local rendering in many cases, and Wayland does not support remote rendering at all). It is obvious the X server is not passing everything through unchanged to pixman, making the idea that local and remote rendering can share code by reusing the pixman api not work in practice. I think it would be a good idea to dump pixman (and remote xlib rendering), moving the code to Cairo xlib backend and (hopefully) cleaning it up.