Drivers for Scene, World, and Compositing Nodes now "work" (well,
sort-of)! Previously they were strictly restricted to object-
accessible data only; now they can function across the board (give or
take some weak spots).
Although there is still no depsgraph support so that these properties
update properly when their source controls are changed (this will
probably require a lot more work), they can still update under other
circumstances (i.e. frame change and/or manual refresh flushing via
mouse movement, etc.)
As the depsgraph tagging support is lacking, these just get always
executed for now, which might potentially be quite sluggish, though it
is hoped that there are so few of these top-level datablocks with
drivers hooked up that this is barely an issue in practice. At least I
haven't noticed any substantial slowdowns for animation playback, so
it should probably be fine.
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