minor #23435 [TwigBridge] cleaner implementation of the TwigRenderer (Tobion)

This PR was merged into the 2.7 branch.

Discussion
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[TwigBridge] cleaner implementation of the TwigRenderer

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no <!-- don't forget updating src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget updating UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/23432#discussion_r125880092
| License       | MIT
| Doc PR        |

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Commits
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c7c5ba8 [TwigBridge] cleaner implementation of the TwigRenderer

src/Symfony/Bridge/Twig/Form/TwigRenderer.php

@@ -19,16 +19,19 @@ use Twig\Environment;
  */
 class TwigRenderer extends FormRenderer implements TwigRendererInterface
 {
-    /**
-     * @var TwigRendererEngineInterface
-     */
-    private $engine;
-
     public function __construct(TwigRendererEngineInterface $engine, $csrfTokenManager = null)
     {
         parent::__construct($engine, $csrfTokenManager);
+    }
 
-        $this->engine = $engine;
+    /**
+     * Returns the engine used by this renderer.
+     *
+     * @return TwigRendererEngineInterface The renderer engine
+     */
+    public function getEngine()
+    {
+        return parent::getEngine();
     }
 
     /**
@@ -36,6 +39,6 @@ class TwigRenderer extends FormRenderer implements TwigRendererInterface
      */
     public function setEnvironment(Environment $environment)
     {
-        $this->engine->setEnvironment($environment);
+        $this->getEngine()->setEnvironment($environment);
     }
 }