Currently Spine doesn't have this feature. It's almost a select transparency and then invert, but it needs to stay 2px from non-transparent pixels (to avoid filtering issues) and Spine meshes can't have holes in them. Once we know the pixels that make up the edges, we need to convert that to a polygon. This probably requires a user setting for how detailed you'd like the polygon.
The next best thing is the New button in the Edit Mesh dialog, which lets you quickly create the mesh hull. It is slightly more time consuming, but you are likely to end up with fewer mesh vertices since you can make more intelligent and sophisticated decisions.
