Alternative pixel-based regtest checker

[Phil Holmes]

4 files attached.  To try this out: create a new directory and place 
NoTagline.ly in it.  Create a subdirectory called input and put some regtest 
files in there.  Run MakeOldPix.sh.  Make a change to lilypond.  Run 
MakeNewPix.sh.  Run ComparePix.sh.  Om my tests there were some false 
alarms, but it seems a fairly simple way to make an alternative regtest 
checker.

There's lots of improvements that could be made: parallelism, using 
something other than diff, but it basically works.

Thoughts?

--
Phil Holmes

ComparePix.sh
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MakeNewPix.sh
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MakeOldPix.sh
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NoTagline.ly
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