Issue 4419: Engraving ends too early

Issue 4419: Engraving ends too early

This is a followup on the solution for issue 2010 that was too eager
killing off unrelated iterators when an iterator in the vicinity of a
Lyric_combine_music_iterator died.

The salient point is to have Simultaneous_music_iterator::process_music
check for pending_moment () going from finite to infinite when
iterating, signifying the loss of an iterator defining an end point.

It happens to also fix issue 4339.


Also contains commit:

simplify Simultaneous_music_iterator::ok

[Dan Eble, 2015-07-10 02:56:43]

I haven't absorbed enough of the background knowledge on iteration to say
credibly that this looks good, although the C++ looks fine.

Because this is a second try at fixing a problem, perhaps a regression test
should be written or expanded to cover the case that the first try did not.

[Dan Eble, 2015-07-10 02:58:58]

Also, thank you.

[dak, 2015-07-10 09:44:04]

On 2015/07/10 02:56:43, Dan Eble wrote:
> I haven't absorbed enough of the background knowledge on iteration to say
> credibly that this looks good, although the C++ looks fine.
> 
> Because this is a second try at fixing a problem, perhaps a regression test
> should be written or expanded to cover the case that the first try did not.

Well, I'd like such a test to also cover issue 2608 which is not yet on
countdown.  Making it part of _this_ issue would be a bad idea since issue 2608
is a crash, and a regtest for a crash should not be committed before the fix.