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.

Tested against samples `simple-face.rb`, `expert-tankspank.rb`,
`expert-minesweeper-adjusted.rb` and `good-clock.rb` (all of which involve
some meaningful line drawing).