Make requestAnimationFrame callback execution easier to reason about

[jdm]

#17398 works around a problem where our short-circuiting layout behaviour gets in the way of our animation callbacks. When layout does not run, it does not generate a frame for the compositor, so the compositor code does not have a chance schedule further animation callbacks. The solution in that PR is to force a real layout to occur in this case, which is self-contained but not great. I see two possible alternatives:

• make layout operations for requestAnimationFrame impossible to short-circuit
• make the compositor schedule the animation callbacks independently of the actual compositing operation in headless mode