#2884 Spanners should extend into \alternative blocks automatically

[Anonymous]

Originally created by: *anonymous

Originally created by: tdanielsmusic

Ties, slurs, glissandi and other spanners extending into \alternative
blocks only work correctly for the first alternative.

David Kastrup suggested:

"It might make sense if the repeat constructs siphoned off unfinished
spanners at the start of the first alternative and replicated them as
post-break spanners into every other alternative."

For background, see Issue 2591.