* Release properties of unbroken spanners & items after doing
handle_broken_smobs (); should conserve a little memory.
* Fixed handling of orphaned score-elements. Any element that does
not have its Line_of_score as a ancestor in both directions, is silently
removed from the output. This guarantees that programming errors
don't result in coredumps from Dimension_cache::common_refpoint ():
element->line_l () is always a common reference point.
* Bugfix: set visibility-lambda of clef and octavation-8 explicitly
for default case. Now the octavation 8 is not printed at every
barline.