Improve outlining for solid geometry in glTFs

[kring]

Fixes #8689

Adds explicit solid geometry outlining for glTF, roughly following the technique described in this paper: https://www.researchgate.net/publication/220067637_Fast_and_versatile_texture-based_wireframe_rendering

With this technique, we get high quality, anti-aliased, non-depth-fighting edges without any additional draw calls.

The edges to outline are specified using a glTF extension, a rough draft of which is here:
https://github.com/kring/glTF/blob/CESIUM_primitive_outline/extensions/2.0/Vendor/CESIUM_primitive_outline/README.md

In short, the edges are specified using an indexed line list, much like you'd do with a separate GL_LINES primitive. There's a bit of extra work at load time to turn this edge list into additional vertex attributes, and sometimes it even requires duplicating vertices. I initially thought I'd have to do this in a web worker, but the performance seems quite good, so I'm going to try to get away without. Doing it in a web worker would requiring shuffling vertex and index buffers around, which would be a hassle and possibly cost more time than it saves.

Bonus features in this PR:

• Basic support for creating a texture with explicit mipmaps.
• A new GltfBuilder class in the tests, which is a handy way of building glTF models on the fly. It's a bit half-baked right now, but it made the ModelOutlineLoader tests easier to write and (hopefully) understand.

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

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

[likangning93]

@kring some preliminary feedback for the Renderer and Scene changes, mostly either small or philosophical things. From what I can tell this looks pretty solid 👌

I'll take a look at the Specs changes soon.

[kring]

Thanks for the review @likangning93! I think I've addressed everything, except that I need to write a test or two for the index enbiggening.

[likangning93]

Thanks @kring! Just one more place where doc needs updating. I took a look at the specs too, those pretty much look good to me.

[likangning93]

I need to write a test or two for the index enbiggening.

Oof. That sounds like it's going to be an impressive spec.

[kring]

Ready for another look!

[likangning93]

Thanks @kring! The new spec looks great 👍, just one more thing:

[likangning93]

Thanks @kring! I can merge this once CI passes.

[weiyinggh]

@likangning93 Can you provide an example program to realize the effect of contour line in cesium? thank you！