no need to Erase() alpha layer twice

Change-Id: If2a05302ffe8489823ca989144a5e6a8be51f6e2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/183936
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins

vcl/source/outdev/wallpaper.cxx

@@ -117,8 +117,8 @@ void OutputDevice::Erase()
         if ( eRasterOp != RasterOp::OverPaint )
             SetRasterOp( eRasterOp );
     }
-
-    if( mpAlphaVDev )
+    // If we went into the "if ( mbBackground )" section, then we have already erased the mpAlphaDev.
+    else if( mpAlphaVDev )
         mpAlphaVDev->Erase();
 }