Modern browsers respect the EXIF orientation information of images,
applying rotation and/or mirroring as specified in those tags. The
the `width="..."` and `height="..."` tags are to size the image
_after_ applying those orientation transformations.
The `.width` and `.height` properties of libvips' images are _before_
any transformations are applied. Since we intend to use these to hint
to rendering clients the size that the image should be _rendered at_,
change to storing (and providing to clients) the dimensions of the
rendered image, not the stored bytes.
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