Fixes slope errors from incorrect X extents in Beam::print. (issue 5293060)

[k-ohara5a5a]

http://codereview.appspot.com/5293060/diff/2001/lily/beam.cc
File lily/beam.cc (right):

http://codereview.appspot.com/5293060/diff/2001/lily/beam.cc#newcode987
lily/beam.cc:987: Beam::calc_x_span (Grob *me_non_spanner, Grob
*commonx)
Why should the x-span of a line-broken beam depend on whether we
requested that it have consistent slope across the break?
Shouldn't the 'span' of a broken beam always go to the end of the beam,
beyond the last stem, for the purposes used in Beam::print() ?
If not, what use-case will break when we choose consistent-broken-slope
after this patch ?

http://codereview.appspot.com/5293060/