[Denemo-devel] playback drawing optimization

[Richard Shann]

I have put in the first optimization for drawing while playing back. It
comprises remembering where the playhead damage is, and re-drawing only
that.
Still to do: 

      * turn off flashing cursor
      * or optimize cursor draw
      * optimize page flipping (meanwhile: set number of steps to 1 in
        prefs)

Thanks to sanooj for making this possible.

Richard