Gets the CTM at the gs that set SoftMask: the mask's group is painted in that coordinate system, not the
one current when something is painted through it. The identity when there is no soft mask.
public Matrix SoftMaskMatrix { get; }
ISO 32000-2 §11.6.5.1 (Table 142, G): the group's matrix is concatenated to the CTM in effect when the mask was set.