Per the CSS Transforms spec, when one of the matrices for interpolation
is non-invertible (i.e. cannot be decomposed), the animation must fall
back to discrete interpolation.
Previously we would silently drop the transform, producing "none". Now
we correctly snap between the from/to values at the 50% progress mark,
matching the behavior required by the spec and other browsers.
Also propagate to_matrix() errors instead of silently using a partial
result, and use the previously-unused AllowDiscrete parameter.
When a transform animation involves a non-invertible matrix (e.g. a
matrix3d with zero Z-scale), the spec requires falling back to discrete
interpolation. Currently we drop the transform entirely, producing
"none" at all progress values. A following commit will fix this.
