correct hittesting based on the image shape of alternate sprites

[davidbau]

jquery-turtle supports convex hull hit testing, but it only takes advantage of this for the default turtle shape. It should automatically compute the convex hull of an image loaded via the 'wear' command, and use it for "touches" and "enclosedby" tests. The fact that it does not do this has been noticed by high school students.

Doing correct hit testing requires three things:
(1) we need to inspect the image bits when an alternate image is loaded. Not too hard if we have access to the bits (via CORS). The moment to inspect an image is at finishSet, added in e072cd1
(2) when we don't have access to the bits (because it's offdomain without cross-origin sharing headers), we need make a proxy on pencilcode and bounce the request off, so we get access to the bits.
(3) finally we need to do the bit of computational geometry to make the convex hull and load it up. Before adding new code, see convexHull in the jquery-turtle code.