utils: raspberrypi: ctt: dng_load_image: Work with DNG files from Picamera2

The DNG specification is based on the TIFF file format and recommends
storing the raw image data in a SubIFD and the Exif tags in an Exif IFD.
Other options are allowed, even if not recommended, such as storing both
the raw image data and the Exif data in IFD0, as done by the TIFF/EP
specification.

libcamera-apps use pyexiv2 to produce DNG files, following the DNG
recommendation, while applications based on picamera2 use PiDNG, which
adopts the TIFF/EP structure. Why it does so is not currently clear (see
https://github.com/schoolpost/PiDNG/issues/65 for discussions on this
topic), but as files based on the DNG and TIFF/EP variants exist in the
wild, both need to be supported by ctt.

Add code to identify which tags are being used, and then load the
metadata from the correct tags.

Signed-off-by: William Vinnicombe <william.vinnicombe@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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