A copy of an image written whole already was cut to the painted bounds of
its opacity mask even when the mask was then left out, nested too deep or
past the object limit, so the file read back with a cropped image and no
mask. The copy is now cut to its mask only when the mask is written, and
otherwise written whole.
The progress-doc line now gives the measured total for the warped test
image's pieces: about 7 times the image's samples, not twice.