Fixed IS_POSITION_FIXED_SUPPORTED detection's bug in IE7

Fixed 2px extra offset bug in IE<=8, so that the `IS_POSITION_FIXED_SUPPORTED` detection's result could be true in IE7 (although position:fixed in IE7 is slightly unpredictable.)

In IE<=7, the window's upper-left is at 2,2 (pixels) with respect to the true client. Surprisely, in IE8, the window's upper-left is at -2, -2 (pixels), but other elements tested is just right, so we need adjust this. some infomation about this bug:

- https://groups.google.com/forum/?fromgroups#!topic/comp.lang.javascript/zWJaFM5gMIQ
- https://bugzilla.mozilla.org/show_bug.cgi?id=174397