Fix artifacts with Octahedral Projection

[OmarShehata]

@bagnell This fixes the black artifacts I was seeing with unwrapping the octahedral projection. Notice the black lines that become more obvious at lower mips:

This was happening because as you sample pixels at the edge, the linear filtering will take some of the black pixels. Pushing it 1 or half a pixel away causes seams to be visible.

What I do now is have a border that duplicates the last pixel, so when it does do the linear interpolation it uses the correct pixel. This effectively simulates linear filtering on all the pixels but nearest/clamped on border pixels.

The result:

The cost is one more draw call when constructing this. The Apps/Sandcastle/gallery/IBL WebGL 1.html is an example of unwrapping this projection correctly.

[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]

👍