Render images with Alpha (without sun/sky)

We should be able to render an image such that the backgound sky etc is masked out. This is specially needed when we want a render pass just for the important Center Stage element of the scene.

It is also very relevant in the following scenarios:

  1. When we animate camera in Unreal and do not want to export and take the scene to other 3d apps for rendering the Center Stage element.
  2. We have done light linking and want to render multiple passes of the same scene, some in which fog is not obscuring so much
  3. Render weather and general sun/sky separately from other important elements just to have more freedom in Post.
  4. Most other 3d apps render masks by default

Please do consider this feature.