fix: Enable OpenGL depth test and face culling for View Cube

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/ViewportWidget.cpp

@@ -45,6 +45,9 @@ void ViewportWidget::paintGL()
     glViewport(0, 0, width(), height());
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
+    glEnable(GL_DEPTH_TEST);
+    glEnable(GL_CULL_FACE);
+
     QMatrix4x4 model;
     model.translate(panX, panY, zoom);
     model.rotate(xRot / 16.0f, 1, 0, 0);