Feature: Randomize direction of rail vehicle on build based on probability callback. (#11489)

This allows NewGRF authors to indicate that the game should randomly flip rail vehicles on build, without needing to use random bits nor duplicate sprites to handle it themselves.

To use this functionality, test for callback 162 (CBID_VEHICLE_BUILD_PROBABILITY)  and var10 = 0 (values other than 0 are reserved for future use), and return a value between 0 and 100 inclusive.

The return value is a percentage chance of reversing the vehicle. A value of 0 will always build a forward facing vehicle, and 100 will always build a reverse facing vehicle.

src/newgrf_callbacks.h

@@ -282,6 +282,9 @@ enum CallbackID {
 
 	/** Called to determine the engine name to show. */
 	CBID_VEHICLE_NAME                    = 0x161, // 15 bit callback
+
+	/** Called to determine probability during build. */
+	CBID_VEHICLE_BUILD_PROBABILITY       = 0x162, // 15 bit callback
 };
 
 /**