Fix #12497: Add workaround for motion_counter being implemented correctly.

#12229 stopped updating motion_counter for non-engine parts of trains, and in doing so accidentally followed the spec for NewGRF var 46, which breaks NewGRFs that used to... accidentally work.

Make var 46 return motion_counter of the first engine, regardless of self or parent scope. This means var 46 is always in sync with the head engine, and avoids further changes to when motion_counter is updated.

(cherry picked from commit 9539b02455f672e11f3ac32302a00cffa5507770)

src/newgrf_engine.cpp

@@ -680,7 +680,7 @@ static uint32_t VehicleGetVariable(Vehicle *v, const VehicleScopeResolver *objec
 		}
 
 		case 0x46: // Motion counter
-			return v->motion_counter;
+			return v->First()->motion_counter;
 
 		case 0x47: { // Vehicle cargo info
 			/* Format: ccccwwtt