Heavily modified groups need defragmentation to remove empty space in data array

[bodhid]

Edge case but this scenario makes voxel groups grow in file size:

• Lots of voxels in a chunk
• Remove most of them
• Unload and reload the chunk
• fill up the chunk again
• repeat

The voxel editor has defragmentation, check if it works for in-game chunks as well

[bodhid]

Defragmentation is implemented in the editor for now