Allow the full -1.0 .. 1.0 range for the watermark offsets.

For #9651.
We in zoom to display a single image, ensure that the selection is updated.

Not doing that we will keep the last selected image into the selected_images
table and the remove/delete operations will not operate on the currently
displayed image.

For #9431.
Merge branches 'fix-sel-operation-zoom' and 'allow-bigger-watermark-offsets'

* fix-sel-operation-zoom:
  We in zoom to display a single image, ensure that the selection is updated.

* allow-bigger-watermark-offsets:
  Allow the full -1.0 .. 1.0 range for the watermark offsets.