Emit not-quite-cross-staff beams in the right context.

Emit not-quite-cross-staff beams in the right context.

This is related to 1043 and possibly other bugs.  Previously,
if a staff change happened immediately after the termination of
an auto-engraved cross-staff beam, then the beam was parented
to the wrong staff.  Now, every beam is parented to the context
in which it began.