If you had a single connected path linking the source to the
destination but _also_ had a spurious edge elsewhere in the grid, then
the spurious edge would be highlighted as an error, but it wouldn't
inhibit declaring the game complete and showing the victory flash.
Tracks's previous loop detector was very basic, and only bothered to
highlight one loop, even if the player managed to create more than one
at a time. Now we highlight all of them.
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.
