Renamed include/osg/OperationsThread to OperationThread.

Created a new GraphicsThread subclass from OperationThread which allows the
GraphicsContext specific calls to be moved out of the base OperationThread class.

Updated the rest of the OSG to respect these changes.

src/osg/GraphicsContext.cpp

@@ -472,11 +472,11 @@ void GraphicsContext::createGraphicsThread()
 {
     if (!_graphicsThread)
     {
-        setGraphicsThread(new OperationsThread);
+        setGraphicsThread(new GraphicsThread);
     }
 }
 
-void GraphicsContext::setGraphicsThread(OperationsThread* gt)
+void GraphicsContext::setGraphicsThread(GraphicsThread* gt)
 {
     if (_graphicsThread==gt) return;