Save one-polygon strokes to SVG as polygons, not polylines

This prevents a bug where a simplified stroke (containing only one
polygon) was incorrectly saved as a polyline, which meant the stroke was
lost when the document was loaded.

src/domain/UBGraphicsStroke.cpp

@@ -170,6 +170,9 @@ bool UBGraphicsStroke::hasPressure()
         }
     }
 
+    else
+        return true;
+
     return false;
 }