Improve motion path support

The 'auto-reverse' option for the 'rotate' attribute is now supported.
This enabled auto-rotation, and also adds a 180 degree rotation.

We now support the 'accumulate' option for the animateMotion element.
This corresponds to the iterationComposite option for Web Animations.

The Web Animations Polyfill is updated to support iterationComposite for
motion path effects.

The Web Animations Polyfill no longer reports an error when the SVG
transforms for the motion path complete, as we use '' instead of null as
the default transform value.