Gets the image's soft mask: a one-component image (DeviceGray, its own Decode mapping) whose values, 0 to 1, are the opacity
of the image at each point; null when the image has none.
public DisplayImage? SoftMask { get; }
ISO 32000-2 §11.6.5.2 (a soft-mask image, SMask, Table 143) or §7.4.9 and §8.9.5.1 Table 87 (SMaskInData:
the opacity channel of JPEG 2000 data). It maps onto the unit square like the image, at its own width and height, which may
differ from the image's; a backend resamples it to the device, never to the image's size. It overrides any soft mask in the
graphics state for this image (§11.6.4.3).