Implement Animate trait for SVGPathData.

The basic idea is: we normalize |this| and |other| svg paths, and then
do interpolation on the normalized svg paths. The normalization is to
convert relative coordinates into absolute coordinates, so we could do
real number interpolation on each path command directly.

In this patch, we also make |clip-path:path()| animatable.

Differential Revision: https://phabricator.services.mozilla.com/D4786