Splitting otexstream into otexrowstream and otexstream.

Splitting otexstream into otexrowstream and otexstream.

otexstream used to count lines to build a TexRow, and some other things. The new
class otexrowstream has the line counting feature of the previous otexstream
without other stuff. otexstream is now a subclass of otexrowstream that has the
same features as before.

This is preliminary work for extending the cursor<->row tracking to math.