page-breaking: allow ragged pages to be compressed
define-grobs.scm: use a property for TrillPitch offset; issue 3465
Fix spurious error playing initial grace notes (issue 1412)

This happens because the MIDI output starts at tick 0, regardless of
when the first event is. Initial grace notes start start at negative
ticks, so midi_walker::output_event thinks they're out of order. Fixed
by starting at the first event.
Grace notes: only shorten previous note if overlapping

Previously a grace note after a rest shortened the last note before the
rest.

This patch shortens the previous note to the start of the grace note
rather than by the length of the grace note, so notes before rests
aren't affected unless the grace notes are longer than the rest.