Update GTRender to use MapContent, MapViewport and Layer

This is a follow on to:

http://docs.codehaus.org/display/GEOTOOLS/MapContext+Refactor - http://docs.codehaus.org/display/GEOTOOLS/MapContext+Refactor

Now that the MapContent, MapViewport and Layer classes are in place we can make use of them in GTRender (and implementations).

Update internals of GTRenderer implementations to take advantage of new classes

• Waiting on screen map patch
  * Change GTRender interface to expect MapContent (and MapViewport?)
  * Update implementation to use layers() list; it is a CopyOnWriteArray so it is safe
  Delegate to DirectLayer implementation
  * Update gt-swing code to take advantage of new classes