Commit 44e2690ab loosened check_completion's idea of what made a
square count as 'having track in it' for purposes of checking
violations of the row/column counts. Unfortunately, that loosened
notion also applied to the check for the game being complete - so the
game would announce a win as soon as you had every square shaded, even
if you hadn't actually laid all the exact track positions down.
Now we separately count up the number of track-ish squares and the
number of fully completed ones, and use the former for error checking
and the latter for completion checking.
- Count any square as having a track either if the square is marked
as such (rendered as a different background), or if at least one
adjacent edge is marked as containing a segment of train track
(rendered as train tracks if they're placed, else as an '=').
- Do the same counting in rows and columns.
