Revision 197746

1.)  Moved Renderer instantiation from Driver to FOTreeHandler, as it is the only
FOInputHandler subclass concerned with Renderer creation. (MIFHandler and RTFHandler
are hardcoded to a particular renderer-type).

2.)  Added a RendererOverride member variable in FOUserAgent, for use by
AWTRenderer as well as external (i.e. user-defined) renderers.  FOTreeHandler coded
to reference it.

3.)  Flow validity checking which was added yesterday temporarily commented-out;
I misread the content model from the spec (I will have to account for fo:markers
as initial children.)