regtests about very small differencies

[Janek Warchoł]

Hi all,

I'm going to fix an issue where a note is misplaced by about 0.07
staffspace.  I'll add a regtest for this, but how will we make sure
that it won't be overlooked in the future?  When we watch a regtest
comparison, it shows us the output in a quite low-resolution
rasterized form; it will be impossible to spot the difference.  I can
also think of some more issues that i'm planning to fix which involve
such small changes.

cheers,
Janek