Fix qgis#12023 : Prevent AA artifacts on polygon fill with NoPen style

When NoPen option is selected and Anti-Aliasing is enabled, polygons
that share a boundary are bleeding out on each other, leading to an
unwanted artifact. In that situation we prefer to draw a very thin line with the
same color than the brush.

src/core/symbology/qgsfillsymbollayer.cpp

@@ -306,6 +306,13 @@ void QgsSimpleFillSymbolLayer::renderPolygon( const QPolygonF &points, const QVe
 
   applyDataDefinedSymbology( context, mBrush, mPen, mSelPen );
 
+  if ( mStrokeStyle == Qt::NoPen && fillColor.alpha() == 255 )
+  {
+    mPen.setStyle( Qt::SolidLine );
+    mPen.setWidthF( 0.2 );
+    mPen.setColor( fillColor );
+  }
+
   QPointF offset = mOffset;
 
   if ( mDataDefinedProperties.isActive( QgsSymbolLayer::PropertyOffset ) )