Invert First Person Aiming (#870)

- Inverts first person aiming on either axis, for both stick and gyro controls

Could be separated from gyro if desirable, but I think it makes the most sense to have it apply to both

include/dusk/settings.h

@@ -144,6 +144,8 @@ struct UserSettings {
         ConfigVar<bool> freeCamera;
         ConfigVar<bool> invertCameraXAxis;
         ConfigVar<bool> invertCameraYAxis;
+        ConfigVar<bool> invertFirstPersonXAxis;
+        ConfigVar<bool> invertFirstPersonYAxis;
         ConfigVar<float> freeCameraSensitivity;
         ConfigVar<bool> debugFlyCam;
         ConfigVar<bool> debugFlyCamLockEvents;