Canvas cameras are slave cameras instead of scene graph cameras

This was a long-standing issue where Canvas cameras were rendered
multiple times, once per slave camera. In the case of the Classic
pipeline they were being rendered twice.

Making them slave cameras has the issue of them not being shared
across GraphicsContexts anymore, so it's not possible to see Canvas
displays in extra CameraGroup windows. This is fixed by using OSG
context sharing feature and sharing the initial windows' context
with all subsequent windows.

src/Viewer/WindowBuilder.cxx

@@ -249,6 +249,11 @@ GraphicsWindow* WindowBuilder::buildWindow(const SGPropertyNode* winNode, bool i
             return result;
     }
     auto traits = new GraphicsContext::Traits(*defaultTraits);
+
+    // Attempt to share context with the window that was created first
+    if (!wsa->windows.empty())
+        traits->sharedContext = wsa->windows.front()->gc;
+
     int traitsSet = setFromProperty(traits->hostName, winNode, "host-name");
     traitsSet |= setFromProperty(traits->displayNum, winNode, "display");
     traitsSet |= setFromProperty(traits->screenNum, winNode, "screen");

src/Viewer/renderer.cxx

@@ -1106,13 +1106,27 @@ FGRenderer::setEventHandler(FGEventHandler* eventHandler_)
 void
 FGRenderer::addCamera(osg::Camera* camera, bool useSceneData)
 {
-    _viewerSceneRoot->addChild(camera);
+    osg::Camera *guiCamera = getGUICamera(CameraGroup::getDefault());
+    osg::GraphicsContext *gc = guiCamera->getGraphicsContext();
+    camera->setGraphicsContext(gc);
+    if (composite_viewer) {
+        composite_viewer->getView(0)->addSlave(camera, false);
+    } else {
+        viewer->addSlave(camera, false);
+    }
 }
 
 void
 FGRenderer::removeCamera(osg::Camera* camera)
 {
-    _viewerSceneRoot->removeChild(camera);
+    if (composite_viewer) {
+        unsigned int index = composite_viewer->getView(0)
+            ->findSlaveIndexForCamera(camera);
+        composite_viewer->getView(0)->removeSlave(index);
+    } else {
+        unsigned int index = viewer->findSlaveIndexForCamera(camera);
+        viewer->removeSlave(index);
+    }
 }
 
 void