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Instead of tagging the sources, which is insensitive to changes, track the
known failure modes by recording the current fail as an xfail.png
reference. (We also introduce a new.png to track a fresh error, so that
they are not lost in the noise of the old XFAILs and hopefully do not
cause everyone to fret).

As we have removed the XFAIL tagging we find, surprise surprise, that some
tests are now working -- so review all the reference images (as also some
.ref.png now should be .xfail.png).

Note: I've only checked image,pdf,ps,svg. The test surfaces report some
failures that probably need to addressed in source. I've not correct the
changes for win32 and quartz. Nor fixed up the experimental backends.