DPX multi-image support and other improvements

Read the individual commit logs for details, but the quick summary is:

* Big DPX overhaul and multiple bug fixes to allow full support reading and writing multi-image files (used for stereo).
* Tests thereof
* Beef up DPX output where we were not properly setting orientation, image description, and offset fields.

This is "on top of" the "oiiotool --siappend" patch that is under review separately as #657. Feel free to either review it here, or ignore that part.  They go together because I needed the "--siappend" in order to test this patch.