Allow mapping half axes to buttons

This allows to bind pressure sensitive NeGcon buttons to keyboard,
mouse and controller buttons

src/frontend-common/sdl_controller_interface.h

@@ -1,10 +1,10 @@
 #pragma once
-#include "core/types.h"
 #include "controller_interface.h"
+#include "core/types.h"
 #include <array>
 #include <functional>
-#include <vector>
 #include <mutex>
+#include <vector>
 
 union SDL_Event;
 
@@ -27,7 +27,9 @@ public:
   // Binding to events. If a binding for this axis/button already exists, returns false.
   bool BindControllerAxis(int controller_index, int axis_number, AxisCallback callback) override;
   bool BindControllerButton(int controller_index, int button_number, ButtonCallback callback) override;
-  bool BindControllerAxisToButton(int controller_index, int axis_number, bool direction, ButtonCallback callback) override;
+  bool BindControllerAxisToButton(int controller_index, int axis_number, bool direction,
+                                  ButtonCallback callback) override;
+  bool BindControllerButtonToAxis(int controller_index, int button_number, AxisCallback callback) override;
 
   // Changing rumble strength.
   u32 GetControllerRumbleMotorCount(int controller_index) override;
@@ -59,6 +61,7 @@ private:
     std::array<AxisCallback, MAX_NUM_AXISES> axis_mapping;
     std::array<ButtonCallback, MAX_NUM_BUTTONS> button_mapping;
     std::array<std::array<ButtonCallback, 2>, MAX_NUM_AXISES> axis_button_mapping;
+    std::array<AxisCallback, MAX_NUM_BUTTONS> button_axis_mapping;
   };
 
   using ControllerDataVector = std::vector<ControllerData>;