Added support for tracking the time since the last clear() call to enable other

classes to be able to find out how long the current graphics frame has been running.

include/osg/GraphicsContext

@@ -16,6 +16,8 @@
 
 #include <osg/State>
 #include <osg/GraphicsThread>
+#include <osg/Timer>
+
 #include <vector>
 
 namespace osg {
@@ -328,6 +330,8 @@ class OSG_EXPORT GraphicsContext : public Object
           * Note, must only be called from a thread with this context current.*/
         virtual void clear();
         
+        double getTimeSinceLastClear() const { return osg::Timer::instance()->delta_s(_lastClearTick, osg::Timer::instance()->tick()); }
+        
 
         /** Realize the GraphicsContext.*/
         bool realize();
@@ -496,6 +500,7 @@ class OSG_EXPORT GraphicsContext : public Object
         
         ref_ptr<ResizedCallback>            _resizedCallback;
         
+        Timer_t                             _lastClearTick;
 };