* Enable "Experimental Compositors" in preferences, then choose
Realtime GPU execution mode in node editor sidebar.
* Only supports combined pass input and Render Result combined output.
* No viewer nodes, no file output nodes, and no node previews yet.
* Provide render data, node tree and color management directly instead
of going through scene, as these may be modified by the render pipeline.
Also better for cached texture hits this way.
* Change legacy pass type to pass name.
* Skip file output node when not doing final render.
* Gracefully handle incomplete render results.
Brecht Van Lommel