The ustring stores strings in Utf8 format which is variable encoding,
and I was mixing code that assumed that indexes into Utf8 were valid
rune indexes.
This normalizes point to be based on the rune index, not the offset
into the storage, so we need to do some processing to get this
information right. It might make sense perhaps to "Explode" the string
into a List<Rune> rather than try to manipualte the ustring, but this
should work for now.
Also, fixed the positioning code, and the drawing code for this, should
fix the reported issue that came up in a pull request:
https://github.com/migueldeicaza/gui.cs/pull/90
