Bug: updateNeighborsOfChunk reads chunk.isEmpty (undefined) instead of chunk._isEmpty

[tom288]

In src/lib/world.js, updateNeighborsOfChunk reads chunk.isEmpty (no underscore):

var terrainChanged = (!chunk) || (chunk && !chunk.isEmpty)

But Chunk defines the property as _isEmpty (with underscore). chunk.isEmpty is always undefined, so !chunk.isEmpty is always true, making terrainChanged unconditionally true.

This means every empty chunk arrival marks all 26 neighbours as _terrainDirty, causing unnecessary re-meshing. For typical terrain where most chunks above ground are air, this creates a large amount of redundant meshing work during world loading.

Fix:

- var terrainChanged = (!chunk) || (chunk && !chunk.isEmpty)
+ var terrainChanged = (!chunk) || (chunk && !chunk._isEmpty)