OpenGL Renderer: don't create texture in accessOpenGLTexture

Scene2D calls this from the Scene2D render thread. If the call happens
before the texture was created, it may end up getting created in the
Scene2D render thread, which is not what we want.

Just return a null texture if it wasn't created already.

Pick-to: 6.12 6.11 6.8
Change-Id: I76c8e0baa30a4f502be5d6cb1996916192ddbab0
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>