Plot gerber: fix a too zealous plot of negative objects on the silkscreen layers,

when merging layers to silkscreen layers.
From master branch

pcbnew/plot_board_layers.cpp

@@ -207,6 +207,9 @@ void PlotOneBoardLayer( BOARD *aBoard, PLOTTER* aPlotter, PCB_LAYER_ID aLayer,
 
                 // Plot the mask
                 PlotStandardLayer( aBoard, aPlotter, layer_mask, plotOpt );
+
+                // Disable the negative polarity
+                aPlotter->SetLayerPolarity( true );
             }
 
             break;