Comment # 13 on bug 57660 from Ilia Mirkin

A working impl is in mesa-git now.
http://cgit.freedesktop.org/mesa/mesa/commit/?id=0e71c65db0df86401f2caf26209ff73e3715443a

The sampler-cube-array-shadow test fails, but so does sampler-cube-shadow (on
nva0+), so I'm guessing those two failures are highly related.