icy_draw: Grid scrolls horizontally in the wrong direction when zoomed in

[TheGreenHerring]

Version used: icy_draw_windows_0.3.0 from build #77

As expected, you can zoom in far enough on an image to show only a portion of it, and then use either the scroll bars or the preview window's viewfinder to look at other parts of the image. However, if you do this with the grid activated, the grid will move horizontally in the opposite direction from the image (e.g. the grid moves left while the image moves right). By contrast, the grid will move in the same direction as the image when scrolling vertically.