InputCommon: Add XInput2 InputBackend class.

Source/Core/InputCommon/ControllerInterface/Xlib/XInput2.cpp

@@ -66,9 +66,27 @@ constexpr int XINPUT_MAJOR = 2, XINPUT_MINOR = 1;
 
 namespace ciface::XInput2
 {
-// This function will add zero or more KeyboardMouse objects to devices.
-void PopulateDevices(void* const hwnd)
+class InputBackend final : public ciface::InputBackend
 {
+public:
+  using ciface::InputBackend::InputBackend;
+  void PopulateDevices() override;
+};
+
+std::unique_ptr<ciface::InputBackend> CreateInputBackend(ControllerInterface* controller_interface)
+{
+  return std::make_unique<InputBackend>(controller_interface);
+}
+
+// This function will add zero or more KeyboardMouse objects to devices.
+void InputBackend::PopulateDevices()
+{
+  const WindowSystemInfo wsi = GetControllerInterface().GetWindowSystemInfo();
+  if (wsi.type != WindowSystemType::X11)
+    return;
+
+  const auto hwnd = wsi.render_window;
+
   Display* dpy = XOpenDisplay(nullptr);
 
   // xi_opcode is important; it will be used to identify XInput events by
@@ -119,7 +137,7 @@ void PopulateDevices(void* const hwnd)
       }
       // Since current_master is a master pointer, its attachment must
       // be a master keyboard.
-      g_controller_interface.AddDevice(
+      GetControllerInterface().AddDevice(
           std::make_shared<KeyboardMouse>((Window)hwnd, xi_opcode, current_master->deviceid,
                                           current_master->attachment, scroll_increment));
     }