[css-animationworklet] Stateless animators are not allowed to share animator instances

[flackr]

There are several parts of the spec which are incompatible with sharing stateless animator instances:

The creating-animator-instance steps specify that when creating an animator instance the user agent must construct a new instance and add it to the animator instance set.

The running animators section specifies that the set of animator instances represents the set of animations. This implies that if multiple animations were sharing the same instance, it would only have one time and be called once.

It seems we need to separate instances of animators from running animations.