Move most animation processing to script

This is preparation for sharing this code with layout_2020 and
implementing selective off-the-main-thread animations.

We still look for nodes not in the flow tree in the layout thread.

components/layout/lib.rs

@@ -20,7 +20,6 @@ extern crate serde;
 #[macro_use]
 pub mod layout_debug;
 
-pub mod animation;
 mod block;
 pub mod construct;
 pub mod context;