Unify support for line width magic cookies.

They were added for the Gerber plotter but once there there's the
expectation by others that they'll work.

common/plotters/PS_plotter.cpp

@@ -521,6 +521,15 @@ void PS_PLOTTER::SetCurrentLineWidth( int aWidth, void* aData )
 {
     wxASSERT( outputFile );
 
+    if( aWidth == DO_NOT_SET_LINE_WIDTH )
+        return;
+    else if( aWidth == USE_DEFAULT_LINE_WIDTH )
+        aWidth = m_renderSettings->GetDefaultPenWidth();
+    else if( aWidth == 0 )
+        aWidth = 1;
+
+    wxASSERT_MSG( aWidth > 0, "Plotter called to set negative pen width" );
+
     if( aWidth != GetCurrentLineWidth() )
         fprintf( outputFile, "%g setlinewidth\n", userToDeviceSize( aWidth ) );