Interpolate paired matrices, avoid decomposing rest of chain

This is the update for https://github.com/web-animations/web-animations-js/pull/618 since shans pointed out the issues with the former.

- Do you agree the the last div should rotate fine while the third one does not?  It only differs from the third div by splitting up the action over two different animations.

- How badly do you want to cache the decomposed matrices used for interpolation?  I put caching in [another branch](https://github.com/joeytwiddle/web-animations-js/tree/decompose-matrices-individually-2-with-caching) because it wasn't very pretty.