Revert "Did paint a line at a bad position."

This reverts commit 641784506200614f1b4742c64c1147b076ea1436.
Fixed the RectGradientPainter again, the line at the left or top side
should be painted for the first tab if it is at least 2 pixels away from
the border (if it is only one pixel away the line strangely merges with
the border).