Problem scrolling with long image with zoom

[leonbloy]

The problem happens when seeing an image with many rows at high zoom factor.

To test:
- Load any image (24 bits) and resize it to (say) 8x6000 pixels (6000 rows, 8 columns)
- Zoom to: 1200% (with 1600% the problem also happens)
- Scroll to the bottom, dragging the scroll bar.
Problem: The viewer resets the scroll position to the upper zone (perhaps some int32 overflow ?)

Tested with 1.98.8, Win7-32