output-distance: use ImageMagick's compare for visual comparisons
This has several advantages:
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Also works for output not based on Grobs (eg. titles, footnotes, etc.)
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Also works for page layout
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Also catches regressions below the formatting engine (eg. problems with Cairo or Ghostscript)
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Highlights differences per system. Vertical spacing shifts in the first system do not cause diffs across the entire score image
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Obviates code for reading and writing signature files, as well as the clumsy and suboptimal geometric algorithms in output-distance.py to compare signatures