Changed the freetype plugin so it keeps its own ref_ptr<Font> to fonts

that it creates, rather than have osgText::Font::readFontFile() use the
osgDB::Registry object cache.

This has been done to prevent problems when users flush the object cache,
which was causing a dangling pointer.

src/osgText/Font.cpp

@@ -65,7 +65,7 @@ osgText::Font* osgText::readFontFile(const std::string& filename)
     std::string foundFile = findFontFile(filename);
     if (foundFile.empty()) return 0;
 
-    osg::Object* object = osgDB::readObjectFile(foundFile,true);
+    osg::Object* object = osgDB::readObjectFile(foundFile);
 
     // if the object is a font then return it.
     osgText::Font* font = dynamic_cast<osgText::Font*>(object);