Update sky object

[ntessore]

In light of #118 and our discussion, we should rework the sky foreground.

• The background sky should only have a single constant value. I would also suggest that we rename it to a const source.
• There should be a grad source that is a luminosity gradient over the image. With Define kernel constants also in object code #118 it can be centred on the image.

The choice of words here is deliberate: I think that we can get rid of the FOREGROUND category of objects altogether. If these sources are to be in the foreground, they should simply be placed before the first lens, like all other sources. That would make them usable if there ever was the need to lens those.