Auto-prune Empty Chunks in Level Files

* 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.

dev-assets/doodads/objects/exit-flag.js

@@ -4,6 +4,10 @@ function main() {
 	Self.SetHitbox(22+16, 16, 75-16, 86);
 
 	Events.OnCollide(function(e) {
+		if (!e.Settled) {
+			return;
+		}
+
 		if (e.InHitbox) {
 			EndLevel();
 		}