Add glretrace option '--snapshot-format=MD5' that allow write image MD5 ...

...to a text format file

Collecting MD5 is obtained based on raw buffer. Rendering results are reproducd accurately. Creating .txt in place of PNG Format, that would save more disk space and run faster.
Reference command: glretrace  --snapshot-format=MD5 -s  /tmp/MD5.log  smokinguns.trace.