Another attempt to fix paste/drop positions for good.

Paste/drop target is either the selected area of a drawable, the
drawable itself, or the image:

- if the paste is larger than the target, center on the target
- if there is a viewport, and the viewport intersects with the
  target, center on the intersection
- otherwise, center on the target

Finally, if we did viewport-centered pasting, make sure the paste is
as completely within image bounds as possible.