FGViewer fixes

- ensure active material are cached
 - clear osgDB object cache on exit explicitly

src/Viewer/fgviewer.cxx

@@ -226,5 +226,11 @@ fgviewerMain(int argc, char** argv)
     // pass the loaded scene graph to the viewer.
     viewer.setSceneData(loadedModel.get());
 
-    return viewer.run();
+    int result = viewer.run();
+    
+    // clear cache now, since it contains SimGear objects. Otherwise SG_LOG
+    // calls during shutdown will cause crashes.
+    osgDB::Registry::instance()->clearObjectCache();
+    
+    return result;
 }

utils/fgviewer/fgviewer.cxx

@@ -188,6 +188,7 @@ main(int argc, char** argv)
         SG_LOG(SG_GENERAL, SG_ALERT, "Problems loading FlightGear materials.\n"
                << "Probably FG_ROOT is not properly set.");
     }
+    ml->refreshActiveMaterials();
     simgear::SGModelLib::init(fg_root, props);
 
     // Set up the reader/writer options