NtUserBeginPaint: Taking the bounding box of the update region first and clipping...

NtUserBeginPaint: Taking the bounding box of the update region first and clipping it to the client area afterwards was making rcPaint contain the entire client area when scrolling. It should be done the other way around: clip to the client area first, then box. Since there is already a clipped update region in the DC, this is easy; just use GdiGetClipBox.

svn path=/trunk/; revision=34923