Make Irrlicht identity material const

irr/include/ISceneNode.h

@@ -310,7 +310,11 @@ public:
 	\return The material at that index. */
 	virtual video::SMaterial &getMaterial(u32 num)
 	{
-		return video::IdentityMaterial;
+		// We return a default material since a reference can't be null,
+		// but note that writing to this is a mistake either by a child class
+		// or the caller, because getMaterialCount() is zero.
+		// Doing so will helpfully cause a segfault.
+		return const_cast<video::SMaterial&>(video::IdentityMaterial);
 	}
 
 	//! Get amount of materials used by this scene node.

irr/include/SMaterial.h

@@ -472,7 +472,7 @@ public:
 };
 
 //! global const identity Material
-IRRLICHT_API extern SMaterial IdentityMaterial;
+IRRLICHT_API extern const SMaterial IdentityMaterial;
 
 } // end namespace video
 } // end namespace irr