Materials: Selectively apply appearances

Change the appearance only if the appearance hasn't been set explicitly.
A cached material appearance is used to see if the current appearance
matches the last material. It is also compared against an empty
material to see if the saved material value has been initialized.

This solves the problem of material changes overwriting appearance
changes

src/Gui/ViewProviderGeometryObject.h

@@ -116,6 +116,8 @@ protected:
     SoFCBoundingBox* pcBoundingBox {nullptr};
     SoSwitch* pcBoundSwitch {nullptr};
     SoBaseColor* pcBoundColor {nullptr};
+
+    App::Material materialAppearance;
 };
 
 }  // namespace Gui