Loopy: be friendlier to right-click-less playing style.

Some people don't bother to use the right-click UI action that marks a
line as 'definitely not' rather than the initial default yellow
'unknown'. Previously, Loopy gave those people a UI annoyance for some
classes of mistake they made during solving: it would reliably
highlight a clue square with too _many_ edges around it, but not one
with too few - because in normal right-click-ful play, a clue with too
few LINE_YES only becomes an error when there aren't enough
LINE_UNKNOWN around it to potentially become the remaining YESes in
future.

This change arranges that once the player closes the loop, _then_ we
light up underfilled clues, on the basis that adding any further edge
would be an obvious error, so it's no longer sensible to assume that
the user might be planning to come back and do so.

(It's not a very timely notification of errors - it's easy to imagine
someone making a mistake like this very near the start of play and
only finding out about it when they close the loop at the very end. I
discuss possible improvements in a comment, but I don't think any
improvement avoids that problem completely, so I think it may just be
a form of annoyance that right-click-less players have to live with.)

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