TabBar: changed edge's TabItem ClipRect to not pass an inverted PushClipRect().

While not currently a problem, it would be if ImRect::Overlaps() is changed to use <= instead of < (cc: #3976 which deal with Contains but sort of similar topic).
Changing ImDrawList::PushClipRect()'s intersect_with_current_clip_rect path to use ClipRectFull() would also fix this, but it may ambiguous there which behavior would be correct.
Amend 1ec464eb9.