Proposed PNG eXIf chunk

During January and February 2017 the PNG Development Group discussed
a proposed eXIf chunk for storing the Exif profile. See
http://www.simplesystems.org/png-group/proposals/eXIf/

The PNG group discussion has lapsed without action on the proposal. The proposal
was pretty simplistic: just copy whatever is in a JPEG APP1 chunk, minus
a few overhead bytes, into the eXIf chunk.

I implemented the proposal in GraphicsMagick/coders/png.c
(currently #ifdef'ed out, but can be enabled with -DexIf_SUPPORTED)
to simply copy GraphicsMagick's Exif property, minus the first few bytes,
into an eXIf chunk.

One troublesome aspect was that the GraphicsMagick "Exif" property can contain
the JPEG thumbnail image, which isn't strictly a part of Exif.

I'm thinking of updating the proposal to declare that the thumbnail
should not be
included in the exIf chunk data, and rewriting the ImageMagick chunk support to
eliminate or ignore it.

People wanting to preserve the thumbnail should use a different
private chunk for that.

Thoughts?

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