CameraGroup: Reset compositor order offset on reload

Use the newly added simgear::Compositor::resetOrderOffset() to reset the
order offset counter with each compositor reload. This prevents the
counter increasing indefinitely, which soon results in render orders
beyond those set statically for the GUI and upcoming VR mirror.

src/Viewer/CameraGroup.cxx

@@ -970,6 +970,9 @@ void reloadCompositors(CameraGroup *cgroup)
 {
     cgroup->_viewer->getViewerBase()->stopThreading();
 
+    // Prevent the camera render orders increasing indefinitely with each reload
+    Compositor::resetOrderOffset();
+
     for (auto &info : cgroup->_cameras) {
         // Ignore the GUI camera
         if (info->flags & CameraInfo::GUI)