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.
How's this approach?  turtleNumFrames is number of frames in turtle animation, while decorationAnimationFrames is number of frames in the magic.  Unfortunately they're different to each other in the assets we got, but only by 1.  It means we repeat one frame, which I don't think is noticeable.