VideoBackends:Metal: Cache pipelines

Metal pipelines hold less stuff than Dolphin pipelines, so duplicates will appear

Source/Core/VideoBackends/Metal/MTLVertexFormat.h

@@ -0,0 +1,23 @@
+// Copyright 2022 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <Metal/Metal.h>
+
+#include "VideoBackends/Metal/MRCHelpers.h"
+
+#include "VideoCommon/NativeVertexFormat.h"
+
+namespace Metal
+{
+class VertexFormat : public NativeVertexFormat
+{
+public:
+  VertexFormat(const PortableVertexDeclaration& vtx_decl);
+
+  MTLVertexDescriptor* Get() const { return m_desc; }
+
+  MRCOwned<MTLVertexDescriptor*> m_desc;
+};
+}  // namespace Metal