Fix 1464 (segfault with R1 and metronome)

Fix 1464 (segfault with R1 and metronome)

This code came from Mathias Kilian.

[Graham Percival (old account), 2011-01-02 00:50:13]

Fixes a segfault, passes the regtest comparison, code by Matthias Kilian.

[Neil Puttock, 2011-01-02 17:39:42]

LGTM, but I'd rather we didn't set the parent twice (at least in the default
case where BreakAlignment is acknowledged; obviously once the engraver's moved,
this acknowledger never gets called).

Cheers,
Neil

http://codereview.appspot.com/3858041/diff/3001/input/regression/metronome-mu...
File input/regression/metronome-multimeasure-rest-no-segfault.ly (right):

http://codereview.appspot.com/3858041/diff/3001/input/regression/metronome-mu...
input/regression/metronome-multimeasure-rest-no-segfault.ly:1: \version
"2.13.44"
2.13.45

http://codereview.appspot.com/3858041/diff/3001/input/regression/metronome-mu...
input/regression/metronome-multimeasure-rest-no-segfault.ly:11: \new Staff {
fix indentation

http://codereview.appspot.com/3858041/diff/3001/input/regression/metronome-mu...
input/regression/metronome-multimeasure-rest-no-segfault.ly:24: \consists
"Staff_collecting_engraver"
remove (already present in engraver-init.ly)

[Graham Percival (old account), 2011-01-03 02:55:37]

I've fixed the problems Neil identified in the regtest, but it's still setting
the parent twice, and I'm aware that there's still discussion between Joe and
Matthias about this patch.

I'm mainly putting this on reitveld in case people want to discuss it by adding
comments to places, rather than email.  If it's all wasted effort, no worries.

[Graham Percival (old account), 2011-01-07 08:52:02]

Any other comments about this?  I'm aware of some discussion here about
get_parent:
http://lists.gnu.org/archive/html/lilypond-devel/2011-01/msg00057.html
http://lists.gnu.org/archive/html/lilypond-devel/2011-01/msg00055.html

is this a reason not to accept the patch as it currently stands?  As far as I
can tell, the patch is "good enough", and this would fix a Critical issue, so my
inclination would be to push it.

[Graham Percival (old account), 2011-01-08 06:49:55]

I've done two regtest checks, and it all seems fine, so I've pushed it.