Fix orientation when saving/transforming JPEG images

Summary:
6597d3f3 introduced lossless JPEG transformations including a
reset of the Exif orientation tag before saving a JPEG image.
This leads to wrongly transformed images (modified or unmodified)
if the original JPEG already had an orientation tag with a visible
transformation (not `None` or `Normal`).
This is fixed by not resetting the orientation before saving, which
keeps the original JPEG data including orientation. Only before
executing a lossless transformation the Exif orientation correction
is applied and the tag is reset.

FIXED-IN: 18.08.2

Test Plan:
* Open JPEG image with Exif orientation other than `None`/`Normal`
* Optional: Rotate/Mirror image
* {nav File > Save} or {nav File > Save As}
* Restart Gwenview to clear image cache
* The new image should have the correct orientation

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: rkflx, ngraham

Tags: #gwenview

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