Add support for additive Core Animation tweens.

Authored by featherless on Mon, Mar 20, 5:57 PM.

Description

Add support for additive Core Animation tweens.

Summary:
Additive animations are the preferred mechanism for building cancelable tweens. Any tween type that implements the Subtractable protocol will now be additive by default.

Additive animations are also the default animation type for UIView implicit animations as of iOS 8 or 9 or 10 [citation needed].

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, Material Motion, chuga

Reviewed By: O4 Material Apple platform reviewers, chuga

Subscribers: chuga

Tags: Material Motion

Differential Revision: http://codereview.cc/D2891