Add the ability to add shader hooks to MetaBackgroundActor

Using ClutterEffect is not pratical on MetaBackgroundActor, as the FBO
redirection has a noticeable performance impact. Instead, allow adding
GLSL code directly to the pipeline used to draw the background texture.
At the same time, port MetaBackgroundActor to modern Cogl API.

https://bugzilla.gnome.org/show_bug.cgi?id=669798