cube example: make sure the program is bound

...in paintGL. There is nothing saying the current
program (or other state) cannot change between
paintGL calls (or initializeGL and paintGL calls).

Correctly call program.bind(). This eliminates the
WebGL warning with WebAssembly.

Task-number: QTBUG-111304
Change-Id: I19792f5f193dcdb305473a939780df33deb1f8e1
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
(cherry picked from commit 99c8eca3f4364b1e062684dd71efc8c5c4b9059c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/opengl/cube/mainwidget.cpp

@@ -148,6 +148,7 @@ void MainWidget::paintGL()
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
     texture->bind();
+    program.bind();
 
 //! [6]
     // Calculate model view transformation