D3’s transform interpolator is designed for SVG transforms; unlike CSS3
transforms, SVG transforms do not have units and are limited to 2D. This commit
limits transform interpolation to transition.attr, so that it only applies to
SVG transforms and not CSS3 transforms which are assigned via style. Thus, if
transition.style is used to interpolate the transform style, the default string
interpolator will now be used.

Note that string interpolation of CSS3 transforms is still likely to break; you
should use CSS3 transitions instead. Fixes #1323.