Fix hotspots being shown for a single frame after startup

FGRenderer::update() happens after the CameraGroup is updated, which causes the
Compositor to use the previous frame cull mask. Setting the cull masks
explicitly during FGRenderer::update() fixes the issue.

src/Viewer/CameraGroup.cxx

@@ -816,6 +816,15 @@ CameraGroup* CameraGroup::buildCameraGroup(osgViewer::View* view,
     return cgroup;
 }
 
+void CameraGroup::setCameraCullMasks(osg::Node::NodeMask nm)
+{
+    for (auto& info : _cameras) {
+        if (info->flags & CameraInfo::GUI)
+            continue;
+        info->compositor->setCullMask(nm);
+    }
+}
+
 void CameraGroup::resized()
 {
     for (const auto &info : _cameras)

src/Viewer/CameraGroup.hxx

@@ -176,6 +176,9 @@ public:
      * current viewport.
      */
     void setCameraParameters(float vfov, float aspectRatio);
+    /** Set the cull mask on all non-GUI cameras
+     */
+    void setCameraCullMasks(osg::Node::NodeMask nm);
     /** Update camera properties after a resize event.
      */
     void resized();

src/Viewer/renderer.cxx

@@ -854,15 +854,14 @@ FGRenderer::update( ) {
                                  l->get_sun_angle()*SGD_RADIANS_TO_DEGREES);
         _updateVisitor->setVisibility(actual_visibility);
         simgear::GroundLightManager::instance()->update(_updateVisitor.get());
+    }
+
     osg::Node::NodeMask cullMask = ~simgear::LIGHTS_BITS & ~simgear::PICK_BIT;
     cullMask |= simgear::GroundLightManager::instance()
         ->getLightNodeMask(_updateVisitor.get());
     if (_panel_hotspots->getBoolValue())
         cullMask |= simgear::PICK_BIT;
-        camera->setCullMask(cullMask);
+    CameraGroup::getDefault()->setCameraCullMasks(cullMask);
-        camera->setCullMaskLeft(cullMask);
-        camera->setCullMaskRight(cullMask);
-    }
 }
 
 void