Add box-shadow inset animation support for IE

[MattyBalaam]

I have spotted this on two versions: IE9 and IE11, but it works as expected on Firefox and Chrome.

http://codepen.io/MattyBalaam/pen/LEZRWY

Just spotted this in the source:

Todo: Add support for inset boxShadows. (webkit places it last whereas IE places it first.)

[julianshapiro]

Yo! I'll rework this issue into a feature request :)

[MattyBalaam]

Thanks. In case anyone needs a quick and dirty solution to this before the enhancement you can do some browser checking and animate boxShadowY instead.

[Rycochet]

It's lost the relevant source line, but this still occurs in Edge