* 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.
* Touching "fire" pixels in a level will pop up the End Level alert box
saying you've died by fire and can restart the level.
* Update level.WriteFile() to prune broken links between actors before
save. So when a linked actor is deleted, the leftover link data is
cleaned up.
* Slight optimization in Canvas.drawStrokes: if either end of the stroke
is not within view of the screen, don't show the stroke.
