#671 Flipped horizontal mouse-wheel scrolling direction

[dima]

All 3 of (wxt,x11,qt) handle the vertical mouse-wheel properly: wheel-up causes the viewport to move up and the image down. wxt appears to not handle the horizontal wheel at all. x11 and qt do it backwards (wheel-left moves the image, not the viewport). This patch reverses the horizontal wheel behavior to be consistent with the vertical wheel behavior.