Fix flash explanation if it's an int rather than uint
Update ref image after recent 'flash explanation' change
Fix ImageCache::get_pixels() for chbegin != 0 case.

The source data channel offset was being applied using the size of the destination datatype rather than from size of the source datatype.

Fixes #759.
Fix missing return in ImageBugAlgo::flatten().
DPX bug fix -- inappropriate use of "dpx:ImageDescriptor" could make invalid files.

A simple recipe for disaster was
  "oiiotool rgba.dpx --ch R,G,B -o rgb.dpx"
You'd think this would simply strip the alpha from an RGBA file, but
rgba.dpx reported a "dpx:ImageDescriptor" metadata of "RGBA", which when
propagated to rgb.dpx made it produce a file that looked like it still
had 4 channels.  Oops.

So this patch ignores the "dpx:ImageDescriptor" metadata upon output, instead
generating the output descriptor based on the number of channels (and in some
cases, refining if the channel names suggest a specific standard interpretation).
Rebuild PDF docs
CHANGES and bump version for 1.3.10 release
Fix Software attribute for dpx output
Fix exr bug with stringvector attributes in the file
We get the data as std::string[] from IlmImf, but it needs to be char*[] for the
ImageSpec.
Fix build break for OpenEXR 1.x
Fix warning with g++ 4.8 and Boost Python
build warning fixes
CHANGES and bump version for 1.3.11
EXR: Added sxr and mxr as possible extensions