clean up EXR headers

- CODING RULE: never declare static variables in a header! they are
replicated in each object files that includes them.
- "exrCommons.h" could easily be split into parts that go to readExr
and parts that go to writeExr anyway, do got rid of it.
- be compatible with OpenEXR < 1.7.0 (no multiview support) for Ubuntu
13.04 (which has 1.6.1)
update openfx