Corrections to alpha pipeline do_versions

Basically they're aimed to solve issues when scene with sky
was used for compositing. If compo used alpha output result
of current trunk would be completely different form hwo it
was before.

Two heuristics here:
- If there's no world or world color is black, it completely
  equals to straight alpha mode, no further magic is needed
  to preserve compatibility

- If scene is used as Render Layer node and something is
  connected to Alpha output of this node, ensure alpha mode
  for this scene is set to Premultiplied.

Basically it shall give better compatibility and make
4K mango project just happy! :)

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