Codechange: do not use MallocT for the pool

Needed to make the placement new operator use Tindex over size_t because of
ambiguity for the delete operator variant that also has the size.

src/engine.cpp

@@ -610,7 +610,7 @@ void SetupEngines()
 		assert(std::size(mapping) >= _engine_counts[type]);
 
 		for (const EngineIDMapping &eid : mapping) {
-			new (eid.engine.base()) Engine(type, eid.internal_id);
+			new (eid.engine) Engine(type, eid.internal_id);
 		}
 	}
 }