DPX bug fixes:
* Was not handling nonzero image origins properly in DPX read_native_scanline.
* Apparently, some files have 0xffffffff in the 'originalSize' fields
  to indicate that they aren't used.
Accommodate some changes in recent cmake for HDF5 finding
Make ustring implementation safe for libc++.

Don't do the trick where we mess with the std::string internals if it's
using libc++'s headers. Eventually, we'll go back and fix this to do
something even more efficient in that case.
Fix OpenEXR write_scanlines handling of per-channel data types.
Subtle problem that could scramble the buffer.

Also, make 'oiiotool -d' a little less likely to have a latent but
for particular unusual channel orders, when sorting out the -d arguments.
DPX fix: output offset properly