Count MIDI ticks from the beginning of the score, not each staff independently. (issue 73610044)

[dak]

That approach seems too contorted.  The Global context is the one doing
the iteration, so it would appear that it should be responsible for
recording the global starting time if the Midi output needs it.  While
one could argue for storing this in some other context, in cases like
\score { { \skip 1 ... } }
the only actually available context _is_ the Global context if we want
to have the skip make an impact on timekeeping.  So here is where we
need to record the starting time.

https://codereview.appspot.com/73610044/