1.) Moved Renderer instantiation from Driver to FOTreeHandler, as it is the only

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.)

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@197746 13f79535-47bb-0310-9956-ffa450edef68