Source for tag ids was http://www.exiv2.org/tags-panasonic.html.

It appeared all ASCII values in the panasonic maker notes appeared to be
''. Playing around with the offsets showed that this commit fixes this
issue for all my test files.

MakerNoteProbe uses (current) metadata, which at this point only
contains a preliminary placeholder. At the end of the EXIF processing,
this is merged into IFD0, which makes the data available for
MakerNoteProbe too. This commit introduces a EXIF IFD0 merge just before
the maker note is getting probed.

This is not the most elegant way to do this, but by far the easiest.
Better would be to lookup the IFD in the bytebuffer and put the
ExifMetadataReader to work. It seems to me that this involves a lot of
rework in the EXIF code, as it assumes all offsets to be absolute to the
beginning of the file, which is not the case when processing embedded
JPEGs.