Texture changing & Snapshot traversal

In yesterday's meeting, Luke wanted us to improve the 3D knot rendering
engine, mainly two things: 1. to allow user to change the texture of the
strings; 2. when in the snapshot mode, an user should not only be able to play
the animation to see how a knot is tied, but also be able to walk through each
snapshot one by one forwardly and backwardly. 4.5 hours of work today, I have
implemented these two functionalites on the engine.