Fix occasional array bounds JS error in decoration animation

 Fix occasional array bounds JS error in deco animation by setting an initial turtle heading.

Also clean up variable naming (and duplication) around the decoration animation function.
What is turtleNumFrames (which looks like still exists?) vs. decorationAnimationNumFrames? Both of these seem like they might be good candidates to live on the skin (i.e. skin.decorationAnimationNumFrames).