* 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.
