Compositor: Add resetOrderOffset()

To allow for custom compositors and passes created by C++ code alongside
those created by Compositor::create(), add a resetOrderOffset() function
which resets _order_offset back to zero. This will allow FlightGear to
prevent repeated compositor reloads from increasing _order_offset
indefinitely, which soon results in render orders beyond those set
statically for the GUI and upcoming VR mirror compositors.

simgear/scene/viewer/Compositor.hxx

@@ -137,6 +137,7 @@ public:
     Pass *             getPass(const std::string &name) const;
 
     int                getOrderOffset() const { return _order_offset; }
+    static void        resetOrderOffset() { _order_offset = 0; }
 protected:
     osg::View                   *_view;
     osg::GraphicsContext        *_gc;