[css-properties-values-api] `<transform-function>` interpolation between different function types

[lilles]

The spec says that a single <transform-function> also interpolates as the standard transform property value. That would mean that it is possible to interpolate between a translate() and a rotate() with the intermediate values being a matrix(), which is also a valid <transform-function>.

This issue is to confirm if that is the correct interpretation.

While testing Safari, I found that they support interpolation between the same type of function, like translateX() and translateY(), but not between translate() and rotate(), for instance.

@andruud @tabatkins