Add octahedral projection

[OmarShehata]

This is a work in progress. It might need some cleanup like moving the shaders into separate files/style changes, but this takes in an array of cubemaps and packs each into a flat square by flattening an octahedron, and stacks them horizontally.

This is to make the IBL work @bagnell is working on work in WebGL 1.

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[bagnell]

Thanks @OmarShehata! I'm merging and I'll make some edits in the ibl branch.