Interpolating between two transformations expressed using a 3D matrix
can be achieved by decomposing the matrices into their transformations
and do a simple numeric interpolation between the initial and final
states, like we do for other data types.

Luckily for us, the CSS Transforms specification from the W3C provides
the decomposition algorithm, using the "unmatrix" code taken from the
book "Graphics Gems II, edited by Jim Arvo".

Once the matrices have been decomposed, we can simply interpolate the
transformations, and re-apply them onto the result matrix, using the
facilities that Clutter provides for interpolating between two known
GTypes.