fix representation switching from line-position to staff-space (issue 6778050)

[k-ohara5a5a]

The regression tester looks for changed X/Y-extents, but gregorian.ly
enlarges the extents of the BreathingSign so it can be used for these
bar-line-like-things without letting notes overlap, so the regression
tester does not notice these changes.


http://codereview.appspot.com/6778050/diff/1001/lily/breathing-sign.cc
File lily/breathing-sign.cc (left):

http://codereview.appspot.com/6778050/diff/1001/lily/breathing-sign.cc#oldcode88
lily/breathing-sign.cc:88: int const int_dim = (int) ydim[i];
If you want to avoid ending the divisi at a fractional position, maybe
just    ydim[i] = int_dim;

http://codereview.appspot.com/6778050/diff/1001/lily/breathing-sign.cc
File lily/breathing-sign.cc (right):

http://codereview.appspot.com/6778050/diff/1001/lily/breathing-sign.cc#newcode112
lily/breathing-sign.cc:112: assert (line_pos.begin () < it);
Can you assert this here, and similarly a few lines down?   If someone
uses a collapsed staff with all line-positions equal, I would have
thought that lower_bound() would return a pointer to the first
line-position, and similarly upper_bound() a pointer to one beyond the
last line-position.

http://codereview.appspot.com/6778050/