…le(), ellipse(), and inset().

Replace LengthOrPercentage with NonNegativeLengthOrPercentage on
ShapeRadius, Circle, Ellipse. And derive ToAnimatedValue for ShapeSource and
its related types, so we clamp its interpolated results into non-negative
values. (i.e. The radius of circle()/ellipse() and the border-radius of
inset().)

Note: We may get negative values when using a negative easing function, so the
clamp is necessary to avoid the incorrect result or any undefined behavior.

Differential Revision: https://phabricator.services.mozilla.com/D14654