rendencontext: Wallpaper needs to fit the provided dimensions, not the outdev.

Otherwise we are painting according to the rendercontext size in the
double-buffered case.

This fixes the rendering of the image in the startcenter.

Change-Id: I2630137c5d176d818bc1a68a970a9e5256ace97c

vcl/source/outdev/wallpaper.cxx

@@ -177,7 +177,7 @@ void OutputDevice::DrawBitmapWallpaper( long nX, long nY,
     else
     {
         aPos = Point( 0, 0 );
-        aSize = Size( mnOutWidth, mnOutHeight );
+        aSize = Size( nWidth, nHeight );
     }
 
     mpMetaFile = NULL;