Slow SubChunk garbage collection check

[dktapps]

Problem description

PocketMine-MP/src/world/format/SubChunk.php

Line 137 in f7687af

foreach($layer->getPalette() as $p){

getPalette() is expensive on account of building a PHP array on the fly to return to user code.

Proposed solution

This check could just check if getBitsPerBlock() === 0 and if get(0, 0, 0) === emptyBlockId. No foreach required, although collectGarbage() must be called first.

Alternative solutions

I considered that we could add PalettedBlockArray::isUniform(int $value) similar to LightArray, but this seems unnecessary since it's easily doable with user code.