Rewrite host GPU abstraction

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

src/util/cocoa_tools.h

@@ -0,0 +1,11 @@
+// SPDX-FileCopyrightText: 2019-2023 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: (GPL-3.0 OR CC-BY-NC-ND-4.0)
+
+#include <string_view>
+
+#include <Cocoa/Cocoa.h>
+
+namespace CocoaTools {
+  NSString* StringViewToNSString(const std::string_view& str);
+}
+