AnySceneImporter: add a TODO for recognizing `*.mtl`.

While UfbxImporter knows to import these directly, AssimpImporter tries
to load them as FBX and fails (heh!), and ObjImporter has no idea about
materials at all, so recognizing this extension would only add more harm
than good at the moment.

src/MagnumPlugins/AnySceneImporter/AnySceneImporter.cpp

@@ -120,6 +120,9 @@ void AnySceneImporter::doOpenFile(const Containers::StringView filename) {
         plugin = "ModoImporter"_s;
     else if(normalizedExtension == ".ms3d"_s)
         plugin = "MilkshapeImporter"_s;
+    /** @todo pass `*.mtl` files to ObjImporter as well, once the builtin one
+        can handle materials and can open them directly (UfbxImporter can,
+        Assimp tries to open them as a FBX ffs) */
     else if(normalizedExtension == ".obj"_s)
         plugin = "ObjImporter"_s;
     else if(normalizedExtension == ".xml"_s)