Allow music of nominally zero duration to be typeset. (issue 6810087)

[benko . pal]

LGTM in the sense that it won't make things worse; I've tried to
understand the code but failed, see below.


http://codereview.appspot.com/6810087/diff/2001/lily/simple-spacer.cc
File lily/simple-spacer.cc (right):

http://codereview.appspot.com/6810087/diff/2001/lily/simple-spacer.cc#newcode375
lily/simple-spacer.cc:375: Grob *end_col = dynamic_cast<Item *>
(cols[col_index + 1])->find_prebroken_piece (LEFT);
is there anybody out there understanding this code?  shouldn't next_col
be used instead of cols[col_index + 1] (and this whole block put under
the if (next_col)?)

http://codereview.appspot.com/6810087/