Binding events directly to an external bindable will cause that
bindable to hold a permanent reference to the current object.
We use `GetBoundCopy()` or otherwise a local bindable + `.BindTo()` to create a
weak-referenced copy of the target bindable. When the local bindable's
lifetime expires, so does the external bindable's reference to it.
This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.
Since each control point will call this when the path updates, the previous would correct the start segment 3 times instead of just once.
This fixes that.
Here we produce a local bound copy of the path version, and bind it to update the path type.
This way, if the path version updates (i.e. any control point changes type or position), we check that all control points have a well-defined path.
Additionally, if the control point piece is disposed of, the GB should also swoop up the subscription because of the local bound copy.
There was a bug where if you created a slider, moved the last point, and then added a point such that it became a PerfectCurve, it would fail to recover after becoming a Bezier.
This fixes that.