We now keep track of the further offset that we have accessed (though
not necessarily the furthest offset of data that is unparseable by us).
It sometimes supports debugging efforts to have a ballpark idea of how
much EXIF data there is, rather than just "from the signature to the
end of the file" or however big the EXIF container is in the enclosing
image format. The latter can be buggy or ambiguous.
We've encountered a malformed user-comments value in an image from a
Canon EOS 5D Mark II.
2020/05/02 16:28:58 exifjpeg.ifd: [DEBUG] Parsing IFD [IFD/Exif] (0) at offset (0102).
2020/05/02 16:28:58 exifjpeg.ifd: [DEBUG] Current IFD tag-count: (33)
2020/05/02 16:28:58 main.main: [WARNING] Skipping unparseable undefined tag: [IFD/Exif] (9286)
2020/05/02 16:28:58 exifjpeg.ifd: [DEBUG] Descending to IFD [IFD/Exif/Iop].
2020/05/02 16:28:58 exifjpeg.ifd: [DEBUG] Parsing IFD [IFD/Exif/Iop] (0) at offset (a164).
Fixes#33
