fixes OIIO issue 518, extra_attribs properly responds to iterator calls
DPX: properly set pixel aspect ratio for dpx write
Modernize IBA::compare -- take an ROI and nthreads, work for all data types.
IBA::computePixelStats -- make it work for all pixel types
IBA::isConstantColor -- modernize API, make it work with all types, with ROI.
Also add a unit test for it.
IBA:isConstantChannel -- modernize API, respect ROI, add unit test.
ImageBufAlgo::isMonochrome: modernize API, honor ROI, add unit test
IBA::fixNonFinite: modernize API, honor roi, parallelize.
Merge pull request #570 from lgritz/lg-iba2

Another round of ImageBufAlgo API modernization: compare, computePixelStats, isConstantColor, isConstantChannel, isMonochrome, fixNonFinte
New ImageRec ctr that builds from two exemplars, with options for roi selection
IBA and oiiotool cleanup of add & sub
* Implement IBA::sub
* More robust handling of "full/display" rois in IBAprep
* Refactor oiiotool --add and --sub to allow different image sizes and
  handle pixel and display windows properly.
* Clarify comments
Tests for oiiotool --add and --sub
Merge pull request #572 from lgritz/lg-addsub

Add/subtract cleanup
IBA::resample() and oiiotool --resample

This is similar to resize, but instead of properly filtering, it just
samples (either bilinearly or using only the closest pixel).  This is
much lower quality than a proper resize -- when downsampling, you should
expect to alias, and when upsampling, to either replicate pixels or have
obvious linear artifacts -- but it's also up to 10x faster than a resize
of a substantial factor using a good filter like lanczos3.

For high quality, use resize.  But if you want a low-quality thumbnail
as fast as you can possibly get it, this one is what you want.
Strutil::extract_from_list_string
ImageBufAlgo::clamp and oiiotool --clamp
ImageBufAlgo::rangecompress()/rangexpand() compress the range of data
values in an image by keeping values <= 1.0 as they are but rescaling
values > 1.0 to be on a log scale (with a smooth transition between
them.
oiiotool --rangeexpand, --rangecompress
maketx --hicomp performs hilight compensation, a combination of range
compression/expansion and clamping negative values to zero, surrounding
every resize step.
Merge pull request #569 from lgritz/lg-highlights

Highlight compensation (range compression/expansion) to help resizing of HDR images with high contrast.