Lighting reads blocktypes only for blocks under heightmap.

This should theoretically speed it up, since less data is copied back and forth.

Also implemented a possibly more cache-friendly blocklight starter algorithm (PrepareBlockLight2()), is disabled by default, needs perf testing.
Fixed member construction order.
Fixed link errors
Merge pull request #883 from mc-server/CutOffLighting

Lighting reads blocktypes only for blocks under heightmap.