This leaks the old filesystem.

I think this would be cleaner by simply having a `void MetaFileSystem::Remount(IFileSystem *oldSystem, IFileSystem *newSystem)`, which could just replace all references (instead of a new list here that we have to remember to change if this is incomplete.)

Also, maybe it'd be cleaner to have a `IFileSystem *MetaFileSystem::GetSystem(const std::string &prefix)`, then you wouldn't even need currentUMD, could just track it in one place that's guaranteed to always be correct (I'm not sure what happens in this func right now exactly if the new UMD fails to mount.)

-[Unknown]