Use this method to focus on the fragment.
This prepares the document for drawing at the fragment
After calling this function you can add content overlaying the
fragment. For example you might call Doc.FillRect to
Text is drawn at a height of one point, using a transformation
matrix to scale it up. So, in general, after calling this method,
the Doc.Rect Height
will be 1.0.
may all be changed as a result of calling this method.