Introduce ControllerInterface class, port SDL interface over

This breaks controller support in the SDL frontend until it's ported
over to the common interface.

src/frontend-common/controller_interface.h

@@ -0,0 +1,80 @@
+#pragma once
+#include "common_host_interface.h"
+#include "core/types.h"
+#include <array>
+#include <functional>
+#include <map>
+#include <mutex>
+
+class HostInterface;
+class System;
+class Controller;
+
+class ControllerInterface
+{
+public:
+  enum : int
+  {
+    MAX_NUM_AXISES = 7,
+    MAX_NUM_BUTTONS = 15
+  };
+
+  using AxisCallback = CommonHostInterface::InputAxisHandler;
+  using ButtonCallback = CommonHostInterface::InputButtonHandler;
+
+  ControllerInterface();
+  virtual ~ControllerInterface();
+
+  virtual bool Initialize(CommonHostInterface* host_interface);
+  virtual void Shutdown();
+
+  // Removes all bindings. Call before setting new bindings.
+  virtual void ClearBindings() = 0;
+
+  // Binding to events. If a binding for this axis/button already exists, returns false.
+  virtual bool BindControllerAxis(int controller_index, int axis_number, AxisCallback callback) = 0;
+  virtual bool BindControllerButton(int controller_index, int button_number, ButtonCallback callback) = 0;
+  virtual bool BindControllerAxisToButton(int controller_index, int axis_number, bool direction,
+                                          ButtonCallback callback) = 0;
+  
+  virtual void PollEvents() = 0;
+  virtual void UpdateControllerRumble() = 0;
+
+  // Input monitoring for external access.
+  struct Hook
+  {
+    enum class Type
+    {
+      Axis,
+      Button
+    };
+
+    enum class CallbackResult
+    {
+      StopMonitoring,
+      ContinueMonitoring
+    };
+
+    using Callback = std::function<CallbackResult(const Hook& ei)>;
+
+    Type type;
+    int controller_index;
+    int button_or_axis_number;
+    float value; // 0/1 for buttons, -1..1 for axises
+  };
+  void SetHook(Hook::Callback callback);
+  void ClearHook();
+
+protected:
+  System* GetSystem() const;
+  Controller* GetController(u32 slot) const;
+  bool DoEventHook(Hook::Type type, int controller_index, int button_or_axis_number, float value);
+
+  void OnControllerConnected(int host_id);
+  void OnControllerDisconnected(int host_id);
+
+  CommonHostInterface* m_host_interface = nullptr;
+
+  std::mutex m_event_intercept_mutex;
+  Hook::Callback m_event_intercept_callback;
+};