Fixed some "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. Additionally, although this fixes the initial "invalid drawable" messages on startup, they still appear later on when switching between fullscreen and not-fullscreen. This goes some way to addressing GafferHQ#396, but I suspect it'll be very hard to fix it completely without patching Qt in some way.

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.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 ) :