Add AnimationEffect.composite

[steveblock]

Both KeyframeAnimationEffect and PathAnimationEffect have a composite
attribute. This change adds the attribute to their common base class,
AnimationEffect.

[birtles]

This was deliberate because in the model the composition operation is specific to each type of animation effect since we imagined there would be types of effects where addition didn't make sense. Do you think it's better to put it on the base class and split it off later if necessary?
(Note that the meaning is slightly different since in KeyframeAnimationEffect this member represents the 'default' composition operation)

[steveblock]

This was deliberate because in the model the composition operation is specific to each type of animation effect
since we imagined there would be types of effects where addition didn't make sense.
Makes sense, but I'm not sure why this applies to composite but not accumulate (which is on AnimationEffect)?

Do you think it's better to put it on the base class and split it off later if necessary?
Not sure, but it seems odd that it's inconsistent with accumulate.

(Note that the meaning is slightly different since in KeyframeAnimationEffect this member represents the 'default'
composition operation)
True. In fact, I think the latest proposal for the new keyframe syntax removes composite from KeyframeAnimationEffect and establishes a default value for the per-keyframe composite.

WDYT?

[steveblock]

The plan is to move the accumulate attribute to the derived classes to match composite - https://www.w3.org/Bugs/Public/show_bug.cgi?id=22440