md5: Add MD5 library.

From http://www.fourmilab.ch/md5/ .
glretrace: Add '--snapshot-format=MD5' option that allows 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
md5: Fix gcc warning.
retrace: Document MD5 snapshot format.
retrace: Make --snapshot-format=MD5 work as documented.

--snapshot-format is the standard output format. -s is the prefix, not
the actual filename.

That is, for MD5 one should do:

  ./glretrace --snapshot-format=MD5 -s - app.trace
image: Fix bug in RAW image output.
image: Simplify MD5 calculation and output.

No intermediate buffer.