Gets the mask's group as a display list in mask space: one DrawGroup(in DisplayGroup) (isolated, knockout and
colour space from the group's attributes, opacity 1, Normal) holding the group XObject's content, clipped to its bounding box.
Empty when the group paints nothing. null only when IsNone.
public DisplayList Content { get; }
ISO 32000-2 Table 142 (G) and §11.6.6. Built once per mask in a page's list, however many paints use it.