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/PDF_plotter.cpp

@@ -91,9 +91,15 @@ void PDF_PLOTTER::SetCurrentLineWidth( int aWidth, void* aData )
 {
     wxASSERT( workFile );
 
-    if( aWidth == 0 )
+    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 != currentPenWidth )
         fprintf( workFile, "%g w\n", userToDeviceSize( aWidth ) );