More thorough testing showed that Acrobat do not presume that images
fill the page if the CTM is unspecified, as tests/resources/masks.pdf
seems to want. Instead they treat it literally and draw the image
as 1x1 PDF units or 1/72" square in the bottom left corner of the page.
Seems like the best thing to do is ignore any such images for the purpose
of DPI calculation. masks.pdf still works out okay because it has
other images.
For more robustness we could consider invalidating any DPI above some
limit, or warning the user about these microdot thumbnails.
James R. Barlow
9041867f86