create new pixelsToMm() and mmToPixels()
fix bug when \version missing
show margins being used
remove support for multiple pages (now requires LilyPond >= 2.15.41)

Multi-page support was considerably complicating the code, and
the new space-time algorithm will require single pages anyway.
totally revamp synchronization algorithm for significant improvement

We now take a radically different approach to synchronization:
rather than brittle heuristics in ly2video, we tweak the .ly source
so that LilyPond gives us all the data we need for synchronization.
For each grob, it outputs paper positions, timing, and origin within
the source file.  Then for each moment in which one or more grobs
occur, the leftmost grob is used to determine the position at which
the video frame will be focused during that moment.

This is far more robust, so we no longer require special handling
for ties, grace notes, chord names, chords which have note heads
on either side of the stem, etc.
wIP set-global-staff-size 20 for predictable extents
WIP use lilypond line numbers which coincide with emacs
use left-margin as offset for calculating grob X positions
DEBUG width