Cope with scene widgets being destroyed in a weird order

I can't actually test this as the CS still doesn't get far enough with this MR.

components/sceneutil/glextensions.cpp

@@ -4,16 +4,29 @@ namespace SceneUtil
 {
     namespace
     {
-        osg::observer_ptr<osg::GLExtensions> sGLExtensions;
+        std::set<osg::observer_ptr<osg::GLExtensions>> sGLExtensions;
+
+        class GLExtensionsObserver : public osg::Observer
+        {
+        public:
+            static GLExtensionsObserver sInstance;
+
+            void objectDeleted(void* referenced) override
+            {
+                sGLExtensions.erase(static_cast<osg::GLExtensions*>(referenced));
+            }
+        };
+
+        GLExtensionsObserver GLExtensionsObserver::sInstance{};
     }
 
     osg::GLExtensions& getGLExtensions()
     {
-        if (!sGLExtensions)
+        if (sGLExtensions.empty())
             throw std::runtime_error(
                 "GetGLExtensionsOperation was not used when the current context was created or there is no current "
                 "context");
-        return *sGLExtensions;
+        return **sGLExtensions.begin();
     }
 
     GetGLExtensionsOperation::GetGLExtensionsOperation()
@@ -23,6 +36,7 @@ namespace SceneUtil
 
     void GetGLExtensionsOperation::operator()(osg::GraphicsContext* graphicsContext)
     {
-        sGLExtensions = graphicsContext->getState()->get<osg::GLExtensions>();
+        auto [itr, _] = sGLExtensions.emplace(graphicsContext->getState()->get<osg::GLExtensions>());
+        (*itr)->addObserver(&GLExtensionsObserver::sInstance);
     }
 }