The original implementation always returns Rotate::Rotate3D, but it is
not correct, so we have to rewrite it:
1. If both from value and to value are none, we don't have to convert it
into identity value, so just return None.
2. If one of the value is none, we replace it with an identity value based on
the other one's rotate axis.
3. If we only have 2D rotation, we just animate the <angle>.
4. Otherwise, we do interpolation by 3D rotation.
Differential Revision: https://phabricator.services.mozilla.com/D11247
0b9ecbccfe