ControllerInterface: Implement axis scaling for axis-to-axis mappings

This feature allows us to work around analog stick range issues at the
intercardinal directions in certain titles (e.g. Rockman DASH 2) caused
by modern controllers having a tighter logical range of reporting than
PS1 analog controllers.

src/frontend-common/sdl_controller_interface.cpp

@@ -289,7 +289,8 @@ bool SDLControllerInterface::HandleControllerAxisEvent(const SDL_Event* ev)
   const AxisCallback& cb = it->axis_mapping[ev->caxis.axis];
   if (cb)
   {
-    cb(value);
+    // Apply axis scaling only when controller axis is mapped to an axis
+    cb(std::clamp(it->axis_scale * value, -1.0f, 1.0f));
     return true;
   }
 
@@ -386,3 +387,14 @@ void SDLControllerInterface::SetControllerRumbleStrength(int controller_index, c
       SDL_HapticRumbleStop(haptic);
   }
 }
+
+bool SDLControllerInterface::SetControllerAxisScale(int controller_index, float scale /* = 1.00f */)
+{
+  auto it = GetControllerDataForPlayerId(controller_index);
+  if (it == m_controllers.end())
+    return false;
+
+  it->axis_scale = std::clamp(std::abs(scale), 0.01f, 1.50f);
+  Log_InfoPrintf("Controller %d axis scale set to %f", controller_index, it->axis_scale);
+  return true;
+}