* lily/script-column.cc (before_line_breaking): use stable sorting

* lily/script-column.cc (before_line_breaking): use stable sorting
algorithm to preserve script ordering.
(grob_script_priority_less): new function

* lily/script-engraver.cc (process_music):
* lily/text-engraver.cc (process_acknowledged_grobs):
* lily/fingering-engraver.cc (make_script): don't use direction
for script-priority.

* scripts/ly2dvi.py (run_lilypond): don't exit if LilyPond fails,
but try to assemble the files anyway.

* input/regression/prefatory-empty-spacing.ly: new file.