Fixed QwtPlotRenderer.setDiscardFlag/setLayoutFlag args

Added missing default state argument (boolean=True) to fit the corresponding Qwt C++ method signatures:
http://qwt.sourceforge.net/class_qwt_plot_renderer.html#a33439eb1407f3ba78fdd7b50461bbafc

Author: PierreRaybaut

qwt/plot_renderer.py

@@ -94,7 +94,7 @@ def __init__(self, parent=None):
         QObject.__init__(self, parent)
         self.__data = QwtPlotRenderer_PrivateData()
 
-    def setDiscardFlag(self, flag, on):
+    def setDiscardFlag(self, flag, on=True):
         """
         Change a flag, indicating what to discard from rendering
 
@@ -147,7 +147,7 @@ def discardFlags(self):
         """
         return self.__data.discardFlags
 
-    def setLayoutFlag(self, flag, on):
+    def setLayoutFlag(self, flag, on=True):
         """
         Change a layout flag