Changed the osgText::Font implementation so it used a facade us abstract away

the actual implemention. This has been done so that when a freetype font is
created the implementation can unloaded when the freetype plugin is unloaded
without breaking the main font.

Also add image margin around founds to prevent any image boundaries appearing.

examples/osgtext/osgtext.cpp

@@ -15,6 +15,7 @@
 
 #include <osgDB/ReadFile>
 #include <osgDB/WriteFile>
+#include <osgDB/Registry>
 
 #include <osgProducer/Viewer>
 
@@ -514,6 +515,9 @@ int main( int argc, char **argv )
     // set the scene to render
     viewer.setSceneData(rootNode.get());
 
+
+    osgDB::Registry::instance()->closeLibrary("osgdb_freetype.so");
+
     // create the windows and run the threads.
     viewer.realize(Producer::CameraGroup::ThreadPerCamera);