ios: Do not drop the user-provided default QSurfaceFormat

Task-number: QTBUG-47195
Change-Id: I40bc7be816f5e23a63a2fe6d32842143b52f9d58
Reviewed-by: jian liang <jianliang79@gmail.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>

src/plugins/platforms/ios/qiosintegration.mm

@@ -81,7 +81,7 @@ QIOSIntegration::QIOSIntegration()
     // QPlatformBackingStore.
     qApp->setAttribute(Qt::AA_ShareOpenGLContexts, true);
     // And that context must match the format used for the backingstore's context.
-    QSurfaceFormat fmt;
+    QSurfaceFormat fmt = QSurfaceFormat::defaultFormat();
     fmt.setDepthBufferSize(16);
     fmt.setStencilBufferSize(8);
     QSurfaceFormat::setDefaultFormat(fmt);