Some images have Exif.Photo.PixelXDimension and Exif.Photo.PixelYDimension
metadata tags in addition to Exif.Image.ImageWidth and
Exif.Image.ImageHeight (mainly tiff images). So far, we were not updating
these optional tags, meaning they could get out-of-sync with the actual
dimensions when resizing and then exporting the image.

Since these tags are non essential, we will only update them if they are
already present.
(Backport of @Wormnest's 52fb1e93)