It doesn't actually work with a similar logic to the other iterators,
because it iterates ByEnd but forwards, so the hints and the extents
don't come in a matching order.

To prevent complicating this further, replace it with a new
implementation that does only what the one client expects, and put it
directly in SwTextFormatter replacing the previous integer iterator
m_nHintEndIndex, so that it is created only once.

Change-Id: I144bfcf7e837a4fb0e7ec279edfba4732d0ae897