Fix incomplete EXIF data for RAW files

Summary:
In case of RAW files exiv2 retrieved the EXIF data from the JPEG preview
image previewData. This however contains incomplete EXIF data. Thus
provide exiv2 with the full raw image and let it find the most suitable
EXIF data chunk.

BUG: 346510

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: ngraham

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D21161