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

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 https://github.com/knz/bubbline/pull/9