Before, `update_the_rendering()` would update all animations for all
Documents once per-Document. Apart from being generally wrong, the
specification says this should be done once per-Document. This
theoretically means that `update_the_rendering()` is just doing less
work every time it runs.
In addition:
- Don't redirty animations nodes when running rAF callbacks. They
should already be dirty when animations are updated.
- Perform a microtask checkpoint while updating animations as dictacted
by the specification.
- Update comments to reflect the specification text.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>