tdf#80512 KDE4: fix RTL scrollbar handling

Just invert the scrollbar the same way gtk+ does by checking the
button positions and adjust the drawing style accordingly.

Change-Id: I3475c33e074c1509a73da618a4efa44dc61a1a77
(cherry picked from commit 273df176)
Reviewed-on: https://gerrit.libreoffice.org/18033Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

vcl/unx/kde4/KDESalGraphics.cxx

@@ -475,6 +475,8 @@ bool KDESalGraphics::drawNativeControl( ControlType type, ControlPart part,
             option.sliderValue = sbVal->mnCur;
             option.sliderPosition = sbVal->mnCur;
             option.pageStep = sbVal->mnVisibleSize;
+            if (part == PART_DRAW_BACKGROUND_HORZ)
+                option.upsideDown = sbVal->maButton1Rect.Left() > sbVal->maButton2Rect.Left();
 
             //setup the active control...always the slider
             if (sbVal->mnThumbState & ControlState::ROLLOVER)