rendercontext: Improve comboboxes in the double-buffered toolbar.

Change-Id: I91bfb4ea4672ff09482679f8fcf18a65e5a6ee3c
diff --git a/vcl/source/control/edit.cxx b/vcl/source/control/edit.cxx
index 114bab5..e67f09c 100644
--- a/vcl/source/control/edit.cxx
+++ b/vcl/source/control/edit.cxx
@@ -606,7 +606,7 @@ void Edit::ImplRepaint(vcl::RenderContext& rRenderContext, bool bLayout)
        }
        // draw normal text
        Color aNormalTextColor = rRenderContext.GetTextColor();
        SetClipRegion(aNormalClipRegion);
        rRenderContext.SetClipRegion(aNormalClipRegion);

        if (IsPaintTransparent())
            rRenderContext.SetTextFillColor();
@@ -1027,6 +1027,10 @@ void Edit::ImplClearBackground(vcl::RenderContext& rRenderContext, long nXStart,

void Edit::ImplPaintBorder(vcl::RenderContext& rRenderContext, long nXStart, long nXEnd)
{
    // this is not needed when double-buffering
    if (SupportsDoubleBuffering())
        return;

    Point aTmpPoint;
    Rectangle aRect(aTmpPoint, GetOutputSizePixel());
    aRect.Left() = nXStart;