Implement sky

[osrf-migration]

Original report (archived issue) by Isaac Saito (Bitbucket: 130s).

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See http://answers.gazebosim.org/question/8421/no-skyx-texture-on-gzweb/

One thing I didn't mention in the QA post that I still don't know is that without sky setting, Gzweb showed the color of the sky as white/light gray, while with sky it's blacked out, as pictures below. So I assume it's <sky> tag in SDF that's not yet taken by Gzweb but <background> tag is considered (see the commit that is probably making the difference below).

Without tag:

With tag:

A related ticket in downstream.

[osrf-migration]

Original comment by Isaac Saito (Bitbucket: 130s).

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• Edited issue description