style: Treat 3d translate/scale as 2d if the value can be expressed a…

…s 2d.

For the individual transform properties if they spec a value that can be
expressed as 2d we treat as 2d and serialize accordingly.

We drop Translate::Translate and Scale::Scale, and then rename
Translate::Translate3D as Translate::Translate, Scale::Scale3D as
Scale::Scale. So now we use Translate::Translate to represent 2d and 3d
translation, and Scale::Scale to represent 2d and 3d scale. There is no
difference between 2d and 3d translate/scale in Gecko because we always
convert them into 3d format to layers (on the compositor thread), so this
change makes things simpler.

Differential Revision: https://phabricator.services.mozilla.com/D52931