Exif is rewritten even if unchanged

[GoogleCodeExporter]

In the current implementation, Exif data is always dis- and re-assembled
prior to writing a file. For TIFF, this is not a big issue, since it's
extremely likely to be changed. For JPEG this is not the case, because IPTC
and Comments may be stored in another part of the file. It would be better
if the Exif IFD's were copied verbatim.

Original issue reported on code.google.com by [email protected] on 13 Mar 2007 at 9:59