Introduced new osg::discardDeletedOpenGLObjects() methods, and usage of it in

GrpahicsContext::close() to handle cases where deletingOpenGLObjects is no possible,
such as when GraphicsWindowEmbedded is used.

include/osg/GLObjects

@@ -26,6 +26,12 @@ extern OSG_EXPORT void flushDeletedGLObjects(unsigned int contextID, double curr
   * Note, must be called from a thread which has current the graphics context associated with contextID. */
 extern OSG_EXPORT void flushAllDeletedGLObjects(unsigned int contextID);
 
+/** Discard all deleted OpenGL objects.
+  * Note, unlike flushAllDeletedObjectObjects discard does not 
+  * do any OpenGL calls so can be called from any thread, but as a consequence it 
+  * also doesn't remove the associated OpenGL resource so discard should only be 
+  * called when the associated graphics context is being/has been closed. */
+extern OSG_EXPORT void discardAllDeletedGLObjects(unsigned int contextID);
 
 }