Gets the soft mask the group's result composites through: the one in the graphics state at Do, whose values give
the result's shape or opacity per point; IsNone when there is none (always for the page group
and the implicit group of a fill and stroke, whose paints carry the mask themselves).
public DisplaySoftMask SoftMask { get; }
ISO 32000-2 §11.6.4.3 (NOTE 2: a mask applies to a group as a whole) and §11.6.6: inside the group the soft mask starts as None, so the content is not masked again; the backend's own mask (SetSoftMask(in DisplaySoftMask)) does not apply to the group's result, this one does.