Re: Fixing issue 37 with extra position callback (issue3928041)

[k-ohara5a5a]

Mike, I tried Chopin preludes Opus 28 No. 1 (where I thought there might
be trouble because beams need to cross stems there) and No. 2 (the
classic example that benefits from this fix).
These, and the large score + parts I use for informal checking, look
good.
Hope you can keep weaving it in to the fabric of Lilypond, to cooperate
with the existing code, with help as needed.

http://codereview.appspot.com/3928041/