DPX - add full support of subimages.

DPXInput was mostly right but there were some minor bugs to fix, we
weren't calling the right version of dpx::Reader::ReadBlock to properly
index the right subimage.

DPXOutput needed a pretty extensive overhaul for submages -- it had been
written without subimage support at all.  So needed to tease apart the
header writing stuff that applied to the whole file versus each
individual subimage.  In the end, it was mostly code movement, not a ton
of changes, other than to the high-level logic of dealing with mutiple
subimages in the file.  DPX is one of those formats where you need to
know the information about all subimages when you first open the file
for output.
Bug fixes in libDPX input of multi-image files.

The old code had two entry points for ReadBlock and ReadImage: one that
took the element number, and the other that took a descriptor (that's
DPX spec speak for things like R, G, RGBA, Luma, etc.) and read from the
first, presumably unique, element with that descriptor.  This seems to
made the assumption that if there were multiple subimages in the file,
they would each have unique descriptors. This is obviously not the case,
for example a DPX file may contain subimages for left and right stereo
views, but both may have the same kRGBA descriptor.

Anyway, the "read element" version was implemented by looking up that
element's descriptor, then calling the "read the element with this
descriptor" function. So of course it was wrong.  Thus, any code using
libDPX to read both views of a stereo DPX file would end up reading the
first image twice.

This patch fixes the "read element" to really and truly read the element
(specified by index). And the "read by descriptor", in addition to being
somewhat non-sensical, wasn't needed anywhere, so I just removed it.
iconvert - get 'open' error from the right place
DPX output: fix setting of description, offset, originalsize, orientation.
DPX tests for stereo/multi-image
Merge pull request #658 from lgritz/lg-dpx

DPX multi-image support and other improvements