Viewer: Enlarging/Reducing image should happen around the center of display

[ubacher]

Viewer: Image displayed at 100%.
Place an annotation in the center of the displayed image.
When pressing Ctrl minus to reduce the enlargement the annotation should stay
in the center of the display.

Attachment 1 shows the actual behaviour when the enlarged area is near the middle of the image.
Attachment 2 shows the behaviour if the enlarged area is off-center. As can be seen, the annotation
disappears completely after Ctrl-minus is pressed.

Note: There are of course exceptions when the enlargement would cause only part of the image to be shown as
is the case for spots near the edges.

In other words: behaviour should be as in IM3.

[attachment deleted by admin]