Rewrite host GPU abstraction

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

src/duckstation-nogui/nogui_host.h

@@ -3,7 +3,7 @@
 
 #pragma once
 #include "common/types.h"
-#include "core/system.h"
+
 #include <functional>
 #include <string>
 
@@ -12,9 +12,6 @@ namespace NoGUIHost {
 bool InBatchMode();
 void SetBatchMode(bool enabled);
 
-/// Starts the virtual machine.
-void StartSystem(SystemBootParameters params);
-
 /// Returns the application name and version, optionally including debug/devel config indicator.
 std::string GetAppNameAndVersion();
 
@@ -36,4 +33,4 @@ void PlatformWindowFocusLost();
 void PlatformDevicesChanged();
 bool GetSavedPlatformWindowGeometry(s32* x, s32* y, s32* width, s32* height);
 void SavePlatformWindowGeometry(s32 x, s32 y, s32 width, s32 height);
-} // namespace NoGUIHost
+} // namespace NoGUIHost