Codechange: Use variable storage for GrfProps with cargo-type groups. (#13557)

Slots are only allocated when used instead of being reserved.

Array-based GrfProps are still used when the number of options is more limited.

src/engine_base.h

@@ -80,7 +80,7 @@ struct Engine : EnginePool::PoolItem<&_engine_pool> {
 	 * Used for obtaining the sprite offset of custom sprites, and for
 	 * evaluating callbacks.
 	 */
-	GRFFilePropsBase<NUM_CARGO + 2> grf_prop;
+	VariableGRFFileProps grf_prop;
 	std::vector<WagonOverride> overrides;
 
 	Engine() {}