Added support for OSG_CAMERA_THREADING env variable which is set to

"SingleThreaded" will ensure that OsgCameraGroup runs singe threaded.

src/osgProducer/OsgCameraGroup.cpp

@@ -208,7 +208,9 @@ OsgCameraGroup::~OsgCameraGroup()
 
 void OsgCameraGroup::_init()
 {
-    _thread_model = ThreadPerCamera;
+    const char* str = getenv("OSG_CAMERA_THREADING");
+    if (str && strcmp(str,"SingleThreaded")==0) _thread_model = SingleThreaded;
+    else _thread_model = ThreadPerCamera;
 
     _scene_data = NULL;
     _global_stateset = NULL;