Improve playsink GL context sharing

Configure playsink tried element with the bus of the main pipeline. That tried element can be a gl video sink, which would benefit from being able to propagate context messages to the main pipeline and have other internal pipeline elements configured with it. Having different elements configured with the same GL context allows them to share buffers with video/x-raw(memory:GLMemory) caps and achieving zero-copy.

Thanks to Alicia Boya García aboya@igalia.com for her work co-debugging the issue and contributing to find a solution.