Follow up on the issue described in #398

[adroitwhiz]

As a follow-up to #544, I'm considering moving SVG skin silhouette creation into updateSilhouette. To ensure this doesn't cause any breakage, we need to ensure the issue in #398 has been fixed.

Here are all the call sites of Drawable.isTouching, as far as I can tell:

• isTouchingColor and isTouchingDrawables call _candidatesTouching, which calls updateSilhouette on all "candidate" drawables' skins. However, they do not appear to call updateSilhouette on the passed drawable's skin.
• drawableTouching and pick both call updateSilhouette.
• _getConvexHullPointsForDrawable calls drawable.skin.isTouchingLinear directly, and does not call updateSilhouette.

It may also be fruitful to investigate whether we can throw an error inside isTouching if the silhouette is "dirty" without the extra check slowing things down.