Script for running pixel-based regtest comparisons

I think this is reasonably self explanatory.  Running the script with -o -j9 will create a set of "old" images in build/out-png-check.  Make changes to lily, compile and run the script -n -jX and you'll be told which images are different, and a set of image differences will be in regtest-diffs.

Please review.

I'm aware that this will require updates to the CG, but think those should come after this is accepted.

[PhilEHolmes, 2013-03-16 16:51:31]

Please review.

[lemzwerg, 2013-03-16 19:24:34]

LGTM.  My only (very minor) concern are overlong lines in the script, something
which I consider hard to read.  If possible, stay within the 80 character per
line limit.

[janek, 2013-03-17 16:34:00]

LGTM