Add metal-cpp to externals

externals/metal-cpp/QuartzCore/QuartzCore.hpp

@@ -0,0 +1,28 @@
+//-------------------------------------------------------------------------------------------------------------------------------------------------------------
+//
+// QuartzCore/QuartzCore.hpp
+//
+// Copyright 2020-2023 Apple Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//-------------------------------------------------------------------------------------------------------------------------------------------------------------
+
+#pragma once
+
+//-------------------------------------------------------------------------------------------------------------------------------------------------------------
+
+#include "CAMetalDrawable.hpp"
+#include "CAMetalLayer.hpp"
+
+//-------------------------------------------------------------------------------------------------------------------------------------------------------------