[mythtv] Add IPTC support to MythGallery

[Dave Badia]

I’m working on adding IPTC image metadata support to MythGallery as
Picassa embeds caption data in IPTC format.  Given that the exif
library doesn’t support IPTC, here are 2 possible approaches I thought
of:

1. Replace exif with exiv2: the exiv2 lib (http://www.exiv2.org/) can
read both IPTC and EXIF image metadata tags.  We could add the new
IPTC logic and change the old EXIF logic to use the exiv2 lib and
remove libexif completely.  Breaks backward compatibility but avoids
adding another library dependency.
2. Add exiv2:  Add in support for exiv2 in addition to the current
exif dependency.  Keeps backward compatibility but adds yet more flags
to configure script and is probably adds unnecessary confusion for the
end user trying to figure out which they need.

As a proof of concept, I already wrote a patch for #1, but given the
strong dislike for new lib dependencies in myth I wanted to solicit
feedback on the best approach before submitting anything.

Dave