Keyframe fixes

[mattgperry]

This PR makes a few keyframe fixes:

1. If keyframes in two variants are the same, but the variant has changed, re-run the keyframes animation.

<motion.div
  animate={is ? "a" : "b"}
  variants={{ a: { x: [0,100] }, b: {x: [0,100] }  }}
/>

Fixes #1346

2. If animate contains keyframes, set the initial value/style to the initial keyframe

<motion.div animate={{ opacity: [0, 1] }} />
// <div style="opacity: 0"></div>

3. If initial={false} and animate contains keyframes, set the initial value/style to the final keyframe

<motion.div initial={false} animate={{ opacity: [0, 1] }} />
// <div style="opacity: 1"></div>

[codesandbox-ci]

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Latest deployment of this branch, based on commit 70118d4:

Sandbox	Source
Framer Motion: Simple animation	Configuration
App Store UI using React and Framer Motion	Configuration
Framer Motion: Reorder animation	Configuration
Framer Motion: growing item positionTransition issue	Configuration
Framer Motion: Image lightbox	Configuration
loving-yonath-f7x8b	Issue #1346

[nvh]

Maybe variantDidChange?

[nvh]

Can you explain why we take the last element here?