Corrects beamed cross-staff stems' pure heights (issue 6543046)

[k-ohara5a5a]

LGTM of course


http://codereview.appspot.com/6543046/diff/1/lily/stem.cc
File lily/stem.cc (right):

http://codereview.appspot.com/6543046/diff/1/lily/stem.cc#newcode342
lily/stem.cc:342: // find the offset of various staves as well as the
minimum and maximum offset
How does this "find the offset of various staves"?
Maybe the common_refpoint is the System, in the cases where the current
Stem is attached to a cross-staff Beam,
but then the code is still finding offsets of Stems, note Staves.

http://codereview.appspot.com/6543046/diff/1/lily/stem.cc#newcode351
lily/stem.cc:351: // group where the beam lies
Now you're writing too much comment.  Most people think it is a bad idea
to write what the code does, only the reason why.
// Estimate the Stem to extend to the level of the furthest
// note-head in within its beamed group.  If that head is in
// a different VerticalAxisGroup, the height assumes the
// staff-spacing tentatively assumed before line-breaking.

http://codereview.appspot.com/6543046/