textarea: SetValue with input taller than viewport does not position cursor properly

[knz]

When I call SetValue() with a string that contains more lines than the height of the textarea, the cursor gets moved outside of view.

I then need to use MoveToBegin() to make it visible again.

I believe this is because repositionView() does not clamp the result to the viewport height.

Found while working on knz/bubbline#9

[knz]

cc @maaslalani