AFAICS current cairo versions are stable, so I'm not going to touch them but it'd be a good idea to re-check openvg state when bumping. Right now, cairo[openvg] deps on mesa[gallium]; it should prolly be mesa[openvg] now. Also, the mask should prolly go down to <=1.10.2-r1, as I was actually able to build cairo against current mesa[openvg]. Not sure how to test it, however.
I don't have a way to test it either. If you've got any suggestions, I'd appreciate it.
I don't know how to test it either, you should ask luca.
cairo-1.12.0 has now USE="openvg" unmasked. After brief testing there was nothing obviously broken with it.