Winged repeats at bottom of barline span if last staff invisible

There is a relevant TODO in the code for Barline::isBottom(). I can see it's tricky. I can also get this behavior in score without parts, by hiding the bottom staff of a span. And there's a whole lot of other TODO's and commented out code here, and other known limitations in barline span functionality.

But even a partial fix that recognizes the last staff of a score is bottom will fix the case at hand and is worth doing.

Partial fix here, covers this case at least (just don't get fancy with Hide empty staves or invisible staves in the score):

https://github.com/musescore/MuseScore/pull/4313

Partial fix only, so reopened.

Since it's probably not obvious what case is missed, consider a score for string quartet plus piano, barlines extended through string staves, piano staff below that. Turn on hide empty staves. On any system where the cello staff is hidden, you'll get the same artifact:

Relates to #285237: [EPIC] Invisible staves issues

Here's the rest of the fix (including for the converse issue with the top staff of span):

https://github.com/musescore/MuseScore/pull/4964