Rewrite host GPU abstraction

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

src/duckstation-nogui/win32_nogui_platform.cpp

@@ -134,6 +134,11 @@ bool Win32NoGUIPlatform::CreatePlatformWindow(std::string title)
   return true;
 }
 
+bool Win32NoGUIPlatform::HasPlatformWindow() const
+{
+  return (m_hwnd != NULL);
+}
+
 void Win32NoGUIPlatform::DestroyPlatformWindow()
 {
   if (!m_hwnd)