MToon: fix the direction of uv animation

[0b5vr]

Will resolve #458

The diff explains everything

[0b5vr]

We should explicitly define the direction of uv scrolls in the spec of MToon.

Note that the spec of glTF says that the origin of the UV coordinates corresponds to the upper left of a texture image.

Ref: https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#images
Ref: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/TextureCoordinateTest

Anyway, we are going to follow the implementation in the original MToon for now
(which scrolls backward(?) in s axis, forward(?) in t axis).