Rewrite host GPU abstraction

- Don't have to repeat the same thing for 4 renderers.
 - Add native Metal renderer.

src/util/opengl_loader.h

@@ -0,0 +1,11 @@
+// SPDX-FileCopyrightText: 2019-2022 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: (GPL-3.0 OR CC-BY-NC-ND-4.0)
+
+#pragma once
+
+// Fix glad.h including windows.h
+#ifdef _WIN32
+#include "common/windows_headers.h"
+#endif
+
+#include "glad.h"