tdf#92982 vcl rendercontext: no need to call SetupBuffer() twice

No need to call it in PaintHelper::StartBufferedPaint(), which would
happen only for the root of the paint hierarchy. It's enough to do it in
PaintHelper::DoPaint(), which happens for each widget.

Change-Id: Iaf3306ef746bedbe64be36c4efeae73afd75db2a

vcl/source/window/paint.cxx

@@ -124,8 +124,6 @@ void PaintHelper::StartBufferedPaint()
     pFrameData->mbInBufferedPaint = true;
     m_bStartedBufferedPaint = true;
 
-    SetupBuffer();
-
     // Remember what was the map mode of m_aPaintRect.
     m_aPaintRectMapMode = m_pWindow->GetMapMode();