Previously requestAnimationFrame() callbacks were registered with a
static global RequestAnimationFrameDriver shared between all windows.
This led to callbacks still running after navigating away from a page
(This could be seen with the WASM GoL demo).
This commit moves the RequestAnimationFrameDriver (now
AnimationFrameCallbackDriver) to be a member of the HTML::Window
object, then uses the 'active document' parameter of
run_animation_frame_callbacks() to run only the active callbacks.
