Fixed "invalid drawable" error messages on OS X.

I had hoped that this fix would make the previous commit unnecessary, but testing shows that they address different problems and both are needed.

Fixes GafferHQ#396.

python/GafferUI/GLWidget.py

@@ -178,9 +178,26 @@ def __init__( self, format ) :
 		self.setVerticalScrollBarPolicy( QtCore.Qt.ScrollBarAlwaysOff )
 
 		glWidget = self.__createQGLWidget( format )
-
+
+		# On mac, we need to hide the GL widget until the last
+		# possible moment, otherwise we get "invalid drawable"
+		# errors spewing all over the place. See event() for the
+		# spot where we show the widget.
+		glWidget.hide()
+
 		self.setViewport( glWidget )
 		self.setViewportUpdateMode( self.FullViewportUpdate )
+
+	def event( self, event ) :
+
+		if event.type() == event.Type.PolishRequest :
+			# This seems to be the one signal that reliably
+			# lets us know we're becoming genuinely visible
+			# on screen. We use it to show the GL widget we
+			# hid in our constructor.
+			self.viewport().show()
+
+		return QtGui.QGraphicsView.event( self, event )
 
 	def resizeEvent( self, event ) :