Gets or sets the form matrix, which maps form space to the user space of the content that paints it; the identity when absent or not six numbers.
public Matrix Matrix { get; set; }
ISO 32000-2 §8.10.1, Table 93 (Matrix, default [1 0 0 1 0 0]). For the group of a soft mask, the matrix is
concatenated with the CTM at the gs that set the mask (§11.6.5.1). Setting the identity removes the entry. A mutation:
every use of the form shows the change.