Beef up strutil_test
Fix Strutil::split for the case of non-default maxsplit
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