ProjectedTetraHedraMapper fails on OsX when using offscreen rendering
Simply run volume.py with pvpython as follows:
> pvpython volume.py (uses onscreen)
> pvpython --use-offscreen-rendering volume.py (uses offscreen)
(If !1800 (merged) is merged, then simply run as follows instead)
> pvpython volume.py (uses onscreen)
> pvbatch volume.py (uses offscreen)
The "onscreen" succeeds while offscreen fails to produce a decent /tmp/foo.png
on Ista which is a "El Capitan" with Intel Iris GPU.