Fix #1695: Clef change placed outside score.

Fix #1695: Clef change placed outside score.

* lily/break-align-engraver.cc (acknowledge_break_alignable):

  return if ack'd grob is musical (only applies to MetronomeMark)

* lily/metronome-engraver.cc (acknowledge_break_aligned):

  if a valid break-aligned grob has been ack'd, set non-musical for
  MetronomeMark

* scm/define-grobs.scm (all-grob-descriptions):

  remove default setting for non-musical in MetronomeMark definition

[Neil Puttock, 2011-07-13 21:37:04]

A MetronomeMark represents a special case for a break-alignable grob, since
sometimes it will be aligned on noteheads rather than prefatory material.  In
this case, we don't want it to be acknowledged by the Break_alignment_engraver,
since it will hijack the X-parent before the Metronome_engraver can set it at
the end of the timestep.  This causes the Paper_column_engraver to add the
MetronomeMark to the 'elements list of a NonMusicalPaperColumn.  If this paper
column is loose, its X-extent calculation will be incorrect, since it includes
the extent of the MetronomeMark whose common refpoint is different (a System). 
This results in the column being translated away from the MetronomeMark into the
left margin.

This patch fixes issue 1695 by removing the default setting for 'non-musical and
setting it dynamically in the Metronome_engraver.  Note that this isn't
sufficient in itself since `non-musical' checks use Item::non_musical (), which
recursively checks parents; in the case where a MetronomeMark is effectively
musical (i.e., aligned on a notehead, and parented by a PaperColumn), we also
need to return from the Break_alignment_engraver without potentially creating a
new BreakAlignment and setting the temporary X-parent.

Phew :)

[Carl, 2011-07-14 14:59:10]

LGTM.

Careful thinking about how to handle this.  Great job, Neil!

[Graham Percival (old account), 2011-07-19 22:38:30]

Hi Neil,

Please push this; we'd like to get back to 0 Critical issues.

Cheers,
- Graham

[Neil Puttock, 2011-07-20 14:20:20]

On 2011/07/19 22:38:30, Graham Percival wrote:

> Please push this; we'd like to get back to 0 Critical issues.

I'm working on it now.

Cheers,
Neil

[Neil Puttock, 2011-07-20 14:56:33]

On 14 July 2011 15:59,  <Carl.D.Sorensen@gmail.com> wrote:
> LGTM.
>
> Careful thinking about how to handle this.  Great job, Neil!

Thanks.  It took several drafts before I was happy with the explanation. :)

(pushed: bebd93c2dd0d7363f311d912ec1ed1f7dfcb36ba)

Cheers,
Neil