Resubmit of earlier fix to 16 bit PNG encoding/decoding

endianness.  Previous change introduced a python2-ism
in the new test that broke on python3.
END_PUBLIC

TESTED:
 - passed opensource_build (all clean this time, so didn't
   miss any python3 errors.  phew.)
 - unit tests

*** Reason for rollback ***

Fixing python3 test breakage in original CL.

*** Original change description ***

Automated g4 rollback of changelist 124905782.

*** Reason for rollback ***

Failing in python3 test job on jenkins.

*** Original change description ***

BEGIN_PUBLIC
Fixing incorrect endianness on 16 bit PNG encode & decode.
(fixes #2619)
Change: 124987398