Fix and speed up light in chunk snapshots

[masmc05]

This PR speeds up the creation of chunk snapshots that contain light data and, more importantly, reduces the memory footprint of both the creation process and the stored snapshots themselves (empty DataLayers now store only the preset instead of a byte array). It felt odd that a snapshot holding less information was more expensive than NMS SerializableChunkData#copyOf

As a comparison, on a randomly generated force loaded world (chunks with tile entities ignored, as it would be unfair to SerializableChunkData#copyOf), via chunk.getChunkSnapshot(true, true, true, true)

	main	SerializableChunkData#copyOf	faster-snapshots
time per call	5.54 µs	4.17 µs	2.67 µs
calls per second	180.5k	239.6k	374.4k
KiB per call	122.89 KiB	60.43 KiB	43.38 KiB

	CPU	Alloc
main	3dWgAw1Qyq	BR1g2zpnut
faster-snapshots	aBF66G22GF	CL0VM1RuyP
copyOf	jSsmUz9fFu	EOmMRQE1m6

During consistency testing I also noticed that ChunkSnapshot#getBlockSkyLight didn't match Block#getLightFromSky(), which now would also be fixed by this PR