fix some const correctness

src/Gui/View3DInventorViewer.cpp

@@ -971,7 +971,7 @@ SbBool View3DInventorViewer::hasViewProvider(ViewProvider* pcProvider) const
 SbBool View3DInventorViewer::containsViewProvider(const ViewProvider* vp) const
 {
     SoSearchAction sa;
-    sa.setNode(const_cast<ViewProvider*>(vp)->getRoot());
+    sa.setNode(vp->getRoot());
     sa.setSearchingAll(true);
     sa.apply(getSoRenderManager()->getSceneGraph());
     return sa.getPath() != nullptr;

src/Gui/ViewProvider.h

@@ -128,10 +128,10 @@ public:
     virtual ~ViewProvider();
 
     // returns the root node of the Provider (3D)
-    virtual SoSeparator* getRoot(void){return pcRoot;}
+    virtual SoSeparator* getRoot(void) const {return pcRoot;}
     // return the mode switch node of the Provider (3D)
-    SoSwitch *getModeSwitch(void){return pcModeSwitch;}
-    SoTransform *getTransformNode(){return pcTransform;}
+    SoSwitch *getModeSwitch(void) const {return pcModeSwitch;}
+    SoTransform *getTransformNode() const {return pcTransform;}
     // returns the root for the Annotations.
     SoSeparator* getAnnotation(void);
     // returns the root node of the Provider (3D)

src/Gui/ViewProviderPythonFeature.h

@@ -223,7 +223,7 @@ public:
 
     /** @name Nodes */
     //@{
-    virtual SoSeparator* getRoot() override {
+    virtual SoSeparator* getRoot() const override {
         return ViewProviderT::getRoot();
     }
     virtual SoSeparator* getFrontRoot() const override {

src/Mod/Mesh/Gui/ViewProvider.h

@@ -96,7 +96,7 @@ public:
     virtual ~ViewProviderExport();
 
     virtual QIcon getIcon() const;
-    SoSeparator* getRoot(void){return 0;}
+    SoSeparator* getRoot(void) const {return nullptr;}
     std::vector<std::string> getDisplayModes(void) const;
     const char* getDefaultDisplayMode() const;
 };