InputCommon / DolphinQt / Core: Add a "RelativeMouse" input which provides the raw delta mouse input

Co-authored-by: Jordan Woyak <jordan.woyak@gmail.com>

Source/Core/InputCommon/ControllerInterface/ControllerInterface.h

@@ -32,6 +32,23 @@
 #endif
 #define CIFACE_USE_DUALSHOCKUDPCLIENT
 
+namespace ciface
+{
+// A thread local "input channel" is maintained to handle the state of relative inputs.
+// This allows simultaneous use of relative inputs across different input contexts.
+// e.g. binding relative mouse movements to both GameCube controllers and FreeLook.
+// These operate at different rates and processing one would break the other without separate state.
+enum class InputChannel
+{
+  Host,
+  SerialInterface,
+  Bluetooth,
+  FreeLook,
+  Count,
+};
+
+}  // namespace ciface
+
 //
 // ControllerInterface
 //
@@ -66,6 +83,9 @@ public:
   void UnregisterDevicesChangedCallback(const HotplugCallbackHandle& handle);
   void InvokeDevicesChangedCallbacks() const;
 
+  static void SetCurrentInputChannel(ciface::InputChannel);
+  static ciface::InputChannel GetCurrentInputChannel();
+
 private:
   std::list<std::function<void()>> m_devices_changed_callbacks;
   mutable std::mutex m_callbacks_mutex;
@@ -75,4 +95,37 @@ private:
   std::atomic<float> m_aspect_ratio_adjustment = 1;
 };
 
+namespace ciface
+{
+template <typename T>
+class RelativeInputState
+{
+public:
+  void Update()
+  {
+    const auto channel = int(ControllerInterface::GetCurrentInputChannel());
+
+    m_value[channel] = m_delta[channel];
+    m_delta[channel] = {};
+  }
+
+  T GetValue() const
+  {
+    const auto channel = int(ControllerInterface::GetCurrentInputChannel());
+
+    return m_value[channel];
+  }
+
+  void Move(T delta)
+  {
+    for (auto& d : m_delta)
+      d += delta;
+  }
+
+private:
+  std::array<T, int(InputChannel::Count)> m_value;
+  std::array<T, int(InputChannel::Count)> m_delta;
+};
+}  // namespace ciface
+
 extern ControllerInterface g_controller_interface;