[css-transforms-2] Interpolation of rotate3D should convert to quaternions instead of a matrix.

[mattwoodrow]

The current spec requires that rotate3D should be converted to a matrix (if the rotations are not along the same vector), and then interpolated (by decomposing into quaternions).

Most implementations are just converting rotate3D into quaternions directly and interpolating, and we should probably specify that.

[tabatkins]

Is there an observable difference in the two behaviors?

[BorisChiou]

I am also working on the Gecko bug for this: https://bugzilla.mozilla.org/show_bug.cgi?id=1737209. Gecko also converts the axis, angle into the quaternion vector directly and then apply slerp. We use this way because we would like to avoid the redundant operations, e.g. convert rotate3d() into 3d matrix, and decompose it. Converting the axis, angle pair into quaternion vector looks simpler.

However, quaternion vector works well only when the rotation angle is in (-360deg, 360deg). Gecko has this bug and I'm fixing it.