Fix 2183fd4d: [NewGRF] Use divide instead of right shift for signed numbers. (#9480)

"For negative a, the value of a >> b is implementation-defined (in most implementations, this performs arithmetic right shift, so that the result remains negative)."

src/newgrf_engine.cpp

@@ -1189,7 +1189,7 @@ int GetEngineProperty(EngineID engine, PropertyID property, int orig_value, cons
 	if (callback != CALLBACK_FAILED) {
 		if (is_signed) {
 			/* Sign extend 15 bit integer */
-			return static_cast<int16>(callback << 1) >> 1;
+			return static_cast<int16>(callback << 1) / 2;
 		} else {
 			return callback;
 		}

src/train_cmd.cpp

@@ -363,7 +363,7 @@ int Train::GetCurveSpeedLimit() const
 
 		/* Apply max_speed modifier (cached value is fixed-point binary with 8 fractional bits)
 		 * and clamp the result to an acceptable range. */
-		max_speed += (max_speed * this->tcache.cached_curve_speed_mod) >> 8;
+		max_speed += (max_speed * this->tcache.cached_curve_speed_mod) / 256;
 		max_speed = Clamp(max_speed, 2, absolute_max_speed);
 	}