[3d-transforms] slight tweak to animation rules

[I really should cross-post this to public-fx since they'll eventually take ownership of the transforms specs, but that hasn't happened yet]

This came up in: https://bugs.webkit.org/show_bug.cgi?id=67695

Relevant spec at: http://dev.w3.org/csswg/css3-3d-transforms/#animation

Currently the spec says that transform animations interpolate the parameter values for rotate, rotateX, rotateY and rotateZ. The function rotate3d was left out because it probably doesn't make sense to do that for rotate3d(1, 0, 0, 10deg) -> rotate3d(0, 1, 0, 10deg). But I propose we add another clause for the case where the first three parameters match.

This would allow simple animations when the axis of rotation is not one of X, Y, or Z.