writePaperHeader(): simplify set-global-staff-size calculation
Fix meta-variable in --width help text
add automated intelligent vertical cropping (issue #21)

Generation of video frames by cropping from the original PNG image is
now done more intelligently; the top and bottom of the crop are chosen
in order to centre-align the notation within the video frames.

https://github.com/aspiers/ly2video/issues/21
writePaperHeader(): use larger top/bottom paper margins to avoid cropping issues

If the margins are too small, the ideal cropping boundary box could
extend outside the top or bottom of the PNG.  And now that we have
intelligent vertical cropping, there is no danger of having margins
too big.
safeRun(): show command run in debug output