Fix tooltips positions on pages with a scrollbar.#110

Better viewport detection. There are two bugs in current tooltips: 1) Incorrect tooltip positioning. Can be observed on pages with scrollbars. On http://thejit.org/static/v20/Jit/Examples/Icicle/example2.html: Horizontal placement is wrong (left instead of right): http://wstaw.org/m/2012/07/11/plasma-windowedEt2982.png Vertical placement is wrong (down instead of up): http://wstaw.org/m/2012/07/11/plasma-windowedlx2982.png Vertical placement is also wrong as horizontal in the first screenshot (up, when should be down) if body height is forced to 100% in css. 2) Incorrect tooltip widths: http://wstaw.org/m/2012/07/11/plasma-windowednt2982.png Not affected by this patch. This patch fixes the first bug. After the patch is applied, the tooltip position is correct (bottom-right if possible in the current viewport).