The autosave refactoring that was done in
7adad5aaf5831dc5adcee140f38aacc4a5db2518 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 7adad5aaf5831dc5adcee140f38aacc4a5db2518.