[autotests] Introduce a SceneOpenGL ES test

The SceneOpenGLTest is transformed into a GenericSceneOpenGLTest which
can create either an OpenGL or an OpenGL ES scene based on env variable
which it sets in initTestCase. The env variable to set is passed as a
ctor argument from the SceneOpenGLTest and the new SceneOpenGLESTest.

This allows to easily run the same test code for both our OpenGL and
OpenGL ES compositor.