Add a simple skybox and envmap for use in examples

[alice-i-cecile]

Adding a skybox to the motion_blur example would be nice

I agree. I tried this out, but frankly it looks awful with the skyboxes we have available in tree. It would be nice if we had a simpler sky-and-clouds skybox and envmap.

Originally posted by @aevyrie in #13617 (comment)

This should be easy enough, but we can only use permissively licensed assets.

[andristarr]

Are we okay with CC-BY licensed assets? If yes, where should we credit the author of the skybox asset?

[alice-i-cecile]

CC-BY is great: add it to CREDITS.md.

[aevyrie]

As noted in #13671 (and my original comment):

Ideally we would also have an envmap, so the lighting matches the skybox. You can use https://github.com/pcwalton/gltf-ibl-sampler-egui to generate skyboxes and specular/diffuse envmaps from hdr skyboxes you find online.