Use given size as initial text input scroll container size

- If we've got a size from the caller we might as well use
  it as the scroll container size. It's more generally
  usefull than arbitrary values. Otherwize a multi-line
  text input would always need to have it's size adjusted
  even though it's size was given in the ctor.
Merge pull request #352 from samiamwork/scroll_container_size

Use given size as initial text input scroll container size