Scrollbar: add a GetPos method
TextArea: reimplement the autoscrolling behavior

UpdateControls() was useless so I merged that into DrawInternal() and use a flag to trigger a redraw on the next pass.
Scrollbar: Max is a valid value
TextContainer: fix errant logic in calculating line exclusion/print area

the problem wasn't that the region was too large, it was that the region for the previous line and current line should share a border
TextContainer: committing the debug drawing code

it has proven very helpful in diagnosing problems :p