Found the bug: we didn't properly update a piece of state in a specific (but
not all that unusual) sequence of operations involving geometry shaders.

I have a preliminary series containing the fix at
http://cgit.freedesktop.org/~nh/mesa/log/?h=shader-state

I remember that you mentioned other rendering bugs in Dolphin that showed up
using your automated tests, but I don't think you've filed bug reports for
them. Of course they may well have had the same root cause, but if any of them
persist after the fix for this bug, I appreciate similarly high quality bug
reports in the future :)