Updated FinishSelection in Document.cs to use DrawWithOperator to for…

…ce the use of Operator.Source when committing the selection layer.

Pinta.Core/Classes/Document.cs

@@ -212,7 +212,7 @@ public void FinishSelection ()
 
 		var g = new Cairo.Context (Layers.CurrentUserLayer.Surface);
 		selection.Clip (g);
-		layer.Draw (g);
+		layer.DrawWithOperator (g, Operator.Source, opacity: 1.0, transform: true, clipToPath: null);
 
 		Layers.DestroySelectionLayer ();
 		Workspace.Invalidate ();