Because item pure heights are cached, the logic in Accidental_interface::pure_height doesn't work. …

Because item pure heights are cached, the logic in Accidental_interface::pure_height doesn't work. The possibility to have multiple pure height results is eliminated by the caching.

This patch adds the property recalculate-pure-height to the Item interface
in order to signal that recalculating should be done.  Currently, this is
only relevant for accidentals but in theory could be used for any item.
This way, we can retain the benefits of caching for most items while
recalculating pure heights of items whose height changes either with different
start and end points or whose estimation gets more accurate over the
compilation process.

[MikeSol, 2013-12-23 08:10:23]

Fixes item.cc logic

[Keith, 2013-12-24 23:29:33]

The drawback is that the glyph for each accidental, rather than being looked up
5 times (already bad) would be looked up 14 times with this patch.

The fundamental problem seems to be that that the 'stencil property is set to
ly:accidental-interface::print(), and that print function checks whether the
accidental is in a tie (unbroken across lines) an deletes the Accidental if so. 
 Thus we cannot store the stencil in 'stencil until we are absolutely sure we
want it.

Could you move the check for ties from accidental::print(), and the similar
check in accidental::pure-height(), to a new function linked to
Accidental.after-line-breaking ?

Then accidentals can build their true stencil just once (whether they need it or
not, but better than building it 14 times throwing away 13) without any 'pure'
complication, and the check for ties (not broken across lines) is linked to a
more expected property, 'after-line-breaking.