Fix zoom out behavior #25

[webcarrot]

Fix zoom out ( or after any "pointer" move ) behavior #25

1. calculate current real/visible image offset
2. calculate current relative pointer position against not zoomed image
3. calculate next real/visible image offset
4. calculate next offset

[webcarrot]

If I find time I will add support for pinch in another PR.

[fritz-c]

It was like waking up on Christmas morning when I saw the notification for this in my inbox - I couldn't wait to try it out.

This is a huge improvement over the previous implementation. I was embarrassed that someone could lose sight of the image they were looking at by just zooming out in a certain way. This fixes all that. Thank you.