Simplify heightmap calculation and avoid loading empty sections

[SirYwell]

Overview

Description

There are two relevant changes:

1. Instead of a two-dimensional boolean array, we can use one byte array and mark specific bits per heightmap type. This reduces memory loads and simplifies the skip logic a bit. The skip bitset is also replaced by a simple counting variable, which also means we can avoid the array equals check.
2. Add a method hasNonEmptySection that can return false if the section is present but only contains air. As minecraft already tracks that information, we can use it and avoid unnecessary array comparisons.

In my tests, the time spent in the HeightmapProcessor was 8 times less than before.

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