scroll: don't adjust current viewport if request rect is a negative number
scroll: don't adjust current viewport if request rect is a negative number

If the request number is negative, keep current viewport unchanged
for the flickable. Fix the position problem of logining gmail.


Former-commit-id: b5f63668d067e80aa65c84836aa3f7a2388fb091
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser