Re: Issue 908 in lilypond: ImageMagick `compare' commands need some investigating

[lilypond]

Comment #6 on issue 908 by percival.music.ca: ImageMagick `compare'  
commands need some investigating
http://code.google.com/p/lilypond/issues/detail?id=908

It might also be good to add a pixel-by-pixel comparison.  Perhaps not in  
the main "make check" (unless it's really fast -- do a test!), but  
certainly in the official release build.

To avoid confusing the current regtest comparison, this new one should  
happen in a separate directory.  It might be tricky to select a good  
threshold value, but the goal isn't to catch *all* problems; it's just to  
make it a bit harder for problems to be unnoticed.