NamcoGuncon: Add an option to scale X coordinates

Fixes what appears to be curvature correction at the edges of the
screen.

src/core/namco_guncon.h

@@ -62,6 +62,7 @@ private:
   Common::RGBA8Image m_crosshair_image;
   std::string m_crosshair_image_path;
   float m_crosshair_image_scale = 1.0f;
+  float m_x_scale = 1.0f;
 
   // buttons are active low
   u16 m_button_state = UINT16_C(0xFFFF);