Draft: rotation tool: tune suggested center position

Currently, during the preparation of the rotation tool dialog, the rotation center is placed exactly in the middle of the box.

In the case of a +/-90 degrees rotation, the result is more blurry unless the center is in the corner of a pixle or in the middle of a pixel.

This merge request places the suggested rotation center position always on a pixel corner.

See issue description on: #3030 (closed) Fixes #3030 (closed)