OcclusionQueryNode: add resetting of queries

If there's a big view change then it might not be appropriate to wait
for '_queryFrameCount' till the next query is issued, because then
geometry might pop into the view after '_queryFrameCount'.

This is especially important for applications not having a constant
frame rate, but just issue a new frame on demand.

src/osg/OcclusionQueryNode.cpp

@@ -588,6 +588,12 @@ void OcclusionQueryNode::setQueriesEnabled( bool enable )
     _enabled = enable;
 }
 
+void OcclusionQueryNode::resetQueries()
+{
+   OpenThreads::ScopedLock<OpenThreads::Mutex> lock( _frameCountMutex );
+   _frameCountMap.clear();
+}
+
 // Should only be called outside of cull/draw. No thread issues.
 void OcclusionQueryNode::setDebugDisplay( bool debug )
 {