Serialise BlockSchematicStructure.Unpack's lazy initialisation

[Zaldaryon]

Unpack() initialises the schematic lazily on first use, with an unsynchronised check-then-act:

public void Unpack(ICoreAPI api)
{
    if (blocksByPos == null)
    {
        Init(api.World.BlockAccessor);
        LoadMetaInformationAndValidate(api.World.BlockAccessor, api.World, FromFile);
    }
}

A BlockSchematicStructure instance is shared by every worldgen thread that places that structure, so with MaxWorldgenThreads above 1 two threads can both see blocksByPos null and both run the initialisation.

The more damaging case is quieter. Init assigns blocksByPos partway through its own work, well before LoadMetaInformationAndValidate has run. So a second thread can find blocksByPos non-null, return from Unpack believing the schematic is ready, and then read fields that nothing has populated yet. PathwayStarts is one of those, and reading it early throws.

This puts both entry points behind one per-instance lock.

There is no lock-free fast path on purpose. blocksByPos being non-null does not mean unpacking has finished, so a caller cannot use it to decide the lock is unnecessary. Since the lock is per instance and only contended on the first placements of a given schematic, the cost after warm-up is an uncontended monitor acquire.

Found while running worldgen with several threads. Related but separate: anegostudios/VintageStory-Issues#9871.