Comment 2 for bug 781398

This will take a rewrite of MoveTransition to fix - probably one that changes its signature.

Right now, MoveTransition gets positions from the old and new displayables, and passes them to a factory function that creates a Move object. In the new version, we will instead have to continuously evaluate the position of the two displayables and then interpolate that position, with time warp.

That's a bigger change than I want to do in 6.13, so this will wait for 6.14.

Memo to self: The signature for this should be something like MoveTransition(duration, enter_transform=None, leave_transform=None, time_warp=None, old=False).