Fix small memory leak in GraphicsContext

Release GraphicsHelperInterface instances on destruction.

Task-number: QTBUG-61293
Change-Id: I989aefb9102dd8772288d8a46cd47dbe3d16e8c0
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>

src/render/graphicshelpers/graphicscontext.cpp

@@ -403,6 +403,8 @@ void GraphicsContext::releaseOpenGL()
         m_debugLogger->stopLogging();
         m_debugLogger.reset(nullptr);
     }
+
+    qDeleteAll(m_glHelpers);
 }
 
 // The OpenGLContext is not current on any surface at this point