Fixed zoom out on Windows 8 with Firefox

Firefox receives wheel delta of 0.33 on zoom in and -0.33 on zoom out, which turns into Math.ceil(0.33) == 1 and Math.round(-0.33) == 0. After this fix it returns correctly -1 for zooming out.
Worked fine before and after this fix in Chrome and Internet Explorer.
Merge pull request #1575 from cschwarz/patch-1

Fixed zoom out on Windows 8 with Firefox