NamcoGuncon: Add button to shoot offscreen

src/core/namco_guncon.h

@@ -12,6 +12,7 @@ public:
     Trigger = 0,
     A = 1,
     B = 2,
+    ShootOffscreen = 3,
     Count
   };
 
@@ -66,6 +67,7 @@ private:
   u16 m_button_state = UINT16_C(0xFFFF);
   u16 m_position_x = 0;
   u16 m_position_y = 0;
+  bool m_shoot_offscreen = false;
 
   TransferState m_transfer_state = TransferState::Idle;
 };