* Discovered a bug where if you hit the Undo key to erase pixels and an
entire chunk became empty by it, the chunk would have rendering errors
and show as a solid black square instead of the level wallpaper
showing through.
* Chunks that have no pixels in them are culled from the chunker
immediately when you call a Delete() operation.
* The level file saver also calls a maintenance function to prune all
empty chunks upon saving the file. So existing levels with broken
chunks need only be re-saved to fix them.
Noah
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