Spawn preparing re-lights all spawn chunks needlessly

[madmaxoft]

With the new change in the spawn preparation, it has become faster for the initial world generation, but it's slower for starting an already-generated server, since the chunks are being re-lit needlessly. We need to skip the lighting altogether if the chunk can be loaded with lightmap successfully.

[madmaxoft]

Also, the preparation deadlocks if PregenerateDistance is set to 1

[madmaxoft]

I'm doing this.
First I want to introduce a new function, cWorld::PrepareChunk(), that completely prepares the specified chunk (loads / generates / lights) and then calls the specified callback.
For this to work I need to change the world storage and generator to support callbacks, so I'm adding those first.

Then, the spawn preparation will just use this function together with the callback to continue preparing.

Finally, I'll make that function available to Lua so that ChunkWorx can use it as well.