Description of Problem:

Currently the padding of the canvas is constrained
by the image size, which makes the view on the
image unmoveable when the image itself is smaller
then the visible area (then the image is always in
the center). If the image is larger than the
visible area, no padding at all takes place, thus
it is not possible to move the image further away
from the borders of the visible areas, ie if one
wants to draw something close to the edge of an
image, its impossible to move this area of the
image closer to the center of the visible area,
leading often to incorrect clicks on the scrollbar
instead of on the image and such.

Steps to reproduce the problem:
For small images:
- Create an Image
- Zoom out
- The image will be centered in the view, it will
be impossible to move around

For large images:
- Create an Image
- Zoom into the image
- Move the image with the middle button
- once at the border of the image, it is no longer
possible to move it further, thus making it hard
to work at the edges of the image.

Expected Results:
Gimp shouldn't constraint the position of the
drawable area all that much. If one hits the
border of an image, Gimp should allow to scroll
further over the borders of the image itself, thus
allowing to freely position the drawable area in
the visible-view. For small images gimp shouldn't
just center them, but instead allow them to be
positioned freely on the visible space.

How often does this happen? 

Especially when working in fullscreen (where the
image is often smaller then the visible area) or
when working at the borders of images the
limitation of the scrolling area causes throuble.