XObjectImageMask doesn't require ImageMask to be true and uses ColorSpace and Mask in predicates

[bdoubrov]

As far as I understand, XObjectImageMask corresponds to the case when Image XObject is used as an Image Mask. Thus, I would expect that it has only one permitted value (true) for ImageMask key.

Both Mask and ColorSpace are not permitted in Image masks, which is correctly reflected in the XObjectImageMask type. However, the restriction in ImageMask still uses these keys in the predicates, even if they are no longer present in XObjectImageMask.

Is there a special reason for this?

[faceless2]

ImageMasks also mentioned in #38 - the model has moved on a bit since then, so mainly cross-linking for context and reference.