incomplete.txt

How should we handle incomplete files?
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Reading an incomplete TIFF sets i_incomplete, but no others do.

Should this only occur if a flag is set?

Should there be some indication of how much was read?

Some formats, like compressed BMP don't really have a way to tell.  A
valid BMP might be 100 x 100 and only set 1 pixel in the data.  Though
we can't tell what the non-set pixels should be in this case.