Exercise precise timing

Add a test where conflicting settings are set, fractions of a second
apart. The setting from the animation that started most recently is the
current setting.

Firefox does not use accurate sub-millisecond timing, and renders this
animation incorrectly.

The animation can currently be viewed at
https://rawgit.com/ewilligers/svg-animation/precise-timing/examples/compare.html?name=precise-timing