Alright. I dusted up my local GIT clone of MESA, and started checking out and compiling various versions. So far I only looked at the tags, and found the releases between which the issue was added: mesa-9.2.5 (works fine) to mesa-10.0-rc1 (contains the problem). Now comes the painful process of narrowing down which commit between these two tags implements the problem. It might take some time, but I'll post here the moment I have a result.