[usdviewq] Workaround apparent PySide6 GL bug

Recent versions of PySide6 can apparently leave the
setting of glDepthMask set to false in the GL context.

As a simple workaround we always set glDepthMask to
true before executing GL drawing or picking operations.

Fixes #2996
Fixes #3009

(Internal change: 2330403)

pxr/usdImaging/usdviewq/stageView.py

@@ -1702,6 +1702,9 @@ def paintGL(self):
             viewProjectionMatrix = Gf.Matrix4f(frustum.ComputeViewMatrix()
                                             * frustum.ComputeProjectionMatrix())
 
+            # Workaround an apparent bug in some recent versions of PySide6
+            GL.glDepthMask(GL.GL_TRUE)
+
             GL.glClear(GL.GL_COLOR_BUFFER_BIT|GL.GL_DEPTH_BUFFER_BIT)
 
             # ensure viewport is right for the camera framing
@@ -2175,6 +2178,9 @@ def pick(self, pickFrustum):
         # Need a correct OpenGL Rendering context for FBOs
         self.makeCurrent()
 
+        # Workaround an apparent bug in some recent versions of PySide6
+        GL.glDepthMask(GL.GL_TRUE)
+
         # update rendering parameters
         self._renderParams.frame = self._dataModel.currentFrame
         self._renderParams.complexity = self._dataModel.viewSettings.complexity.value