Ensure global Mutex is initialized before Registry

A Registry instance may be accessed before the global mutex, e.g. here:

e77f50371c/src/osgText/Font.cpp (L49)

This leads to the Mutex being destroyed before the Registry is destroyed.

This causes a crash at exit, as described in #1048.

Fixes #1048.

include/osgDB/Registry

@@ -50,7 +50,8 @@ namespace osgDB {
     The RegisterReaderWriterProxy can be used to automatically
     register at runtime a reader/writer with the Registry.
 */
-class OSGDB_EXPORT Registry : public osg::Referenced
+class OSGDB_EXPORT Registry : osg::depends_on<OpenThreads::Mutex*, osg::Referenced::getGlobalReferencedMutex>,
+                              public osg::Referenced
 {
     public: