improve shape annotation bounding box behavior

[incanus]

Building on #109, we should look into automatically determining the bounding box of shape annotations independent of the existence of the layer, so that the layer knows when to be properly drawn to screen. For now the workaround is to either manually set the bounding box from the same points to be used in the shape layer drawing later, or to manually query the delegate for the shape layer, assign it to layer, and implicitly get the bounding box updated that way.