`PTransf16::largest_moved_point` query

[james-d-mitchell]

Again arising from the python bindings, I'd expect

 PTransf16([1, 3, 2, 255, 10]).largest_moved_point()

to return 4 (the unspecified values as 255) but this returns 15. I can see the rationale for returning 15 but this is somewhat counterintuitive to say that the partial transformation moves a point it is not defined on (i.e. 15 is not in the domain of the transformation, so is what way does the transformation "move" 15?)