The svg:animate element

As of this writing I have support for the following properties of the SVG animate element:

* @attributeName
* @begin = clock-value
* @dur = clock-value
* @end = clock-value
* @fill = (freeze|remove)
* @from = length without units!
* @to = length without units!
* @by = length without units!

I do need to deal with @from/@to values that decrease, and for that matter @by values that are negative.

After that I'll be looking at svg:animateMotion.