Line drawing wasn't taking parent into account

Wasn't right to use the absolute positions, should have been using the
element variety instead.

We don't, however, want the slight pixel counting adjustment, so give
the end back a +1 that the calculated elements don't have. (Running
expert-minesweeper-adjusted.rb shows clearly how current code depends on
the exact placement of the lines.)

Fixes #978.
Merge pull request #985 from shoes/line-placement-fix

Line drawing wasn't taking parent into account