ENGINES: Use MetaEngine consistently for getting autosave slot

The autosave refactoring that was done in
7adad5aaf5 used g_engine for identifying the
autosave slot. This worked for in-game save/load, but doesn't fit when
called from the launcher.

Fix by passing MetaEngine to SaveStateDescriptor ctor and using it for this
query.

Amends 7adad5aaf5.

engines/myst3/metaengine.cpp

@@ -56,7 +56,7 @@ public:
 
 		SaveStateList saveList;
 		for (uint32 i = 0; i < filenames.size(); i++)
-			saveList.push_back(SaveStateDescriptor(i, filenames[i]));
+			saveList.push_back(SaveStateDescriptor(this, i, filenames[i]));
 
 		return saveList;
 	}