[usdviewq] Fix GL error spew on shutdown

Closing the usdview window (via the [X] button, or Alt+F4) resulted in a bunch of GL errors when using PySide2. The errors stem from the GL context being destroyed before the renderer in the stage view widget is shutdown. This change of behavior from PySide to PySide2 resulted in the stageView widget being destroyed before the aboutToQuit handler was invoked. The fix reworks the way usdview shuts down.
- Hoist the main window widget into a class that implements closeEvent, which triggers shutdown of the stage (and stageView).
- Connect closeAllWindows to the File>Quit action, which in turn triggers the main window's closeEvent. Closing the application via the window leads to the same shutdown process.
- Remove existing handler registration to the aboutToQuit signal.

(Internal change: 2055246)

pxr/usdImaging/usdviewq/appController.py

@@ -265,6 +265,22 @@ def blocked(self):
 
         return self._count > 0
 
+class MainWindow(QtWidgets.QMainWindow):
+    "This class exists to simplify and streamline the shutdown process."
+    "The application may be closed via the File menu, or by closing the main"
+    "window, both of which result in the same behavior."
+    def __init__(self, closeFunc, printCloseTiming):
+        super(MainWindow, self).__init__(None) # no parent widget
+        self._closeFunc = closeFunc
+        self._printCloseTiming = printCloseTiming
+
+    def closeEvent(self, event):
+        self._closeFunc()
+
+        with Timer() as t:
+            self.close() # Close the MainWindow widget.
+        if self._printCloseTiming:
+            t.PrintTime('tear down the UI')
 
 class AppController(QtCore.QObject):
 
@@ -405,7 +421,8 @@ def __init__(self, parserData, resolverContextFn):
             # start listening for style-related changes.
             self._setStyleSheetUsingState()
 
-            self._mainWindow = QtWidgets.QMainWindow(None)
+            self._mainWindow = MainWindow(lambda: self._cleanAndClose(),
+                                          self._printTiming)
             self._ui = Ui_MainWindow()
             self._ui.setupUi(self._mainWindow)
 
@@ -845,9 +862,9 @@ def __init__(self, parserData, resolverContextFn):
                 self._toggleStop)
 
             # Setup quit actions to ensure _cleanAndClose is only invoked once.
-            self._ui.actionQuit.triggered.connect(QtWidgets.QApplication.instance().quit)
-
-            QtWidgets.QApplication.instance().aboutToQuit.connect(self._cleanAndClose)
+            # Don't register a handler for the 'aboutToQuit' signal,
+            # since the mainWindow's closeEvent handler triggers _cleanAndClose.
+            self._ui.actionQuit.triggered.connect(QtWidgets.QApplication.instance().closeAllWindows)
 
             self._ui.actionReopen_Stage.triggered.connect(self._reopenStage)
 
@@ -2476,7 +2493,8 @@ def GrabViewportShot(self):
     # File handling functionality =============================================
 
     def _cleanAndClose(self):
-
+        # This function is called by the main window's closeEvent handler.
+
         self._settings2.save()
 
         # If the current path widget is focused when closing usdview, it can
@@ -2502,15 +2520,9 @@ def _cleanAndClose(self):
         if self._timer.isActive():
             self._timer.stop()
 
-        # Close the stage.
+        # Close stage and release renderer resources (if applicable).
         self._closeStage()
 
-        # Tear down the UI window.
-        with Timer() as t:
-            self._mainWindow.close()
-        if self._printTiming:
-            t.PrintTime('tear down the UI')
-
     def _openFile(self):
         extensions = Sdf.FileFormat.FindAllFileFormatExtensions()
         builtInFiles = lambda f: f.startswith(".usd")