Make Cesium's shaders editable with the SpectorJS shader editor

[kring]

Live, on-the-fly editing of shaders. Next best thing to an actual shader debugger.

I hacked up CesiumJS to make this work while I was working on the log depth stuff, and found it super useful. So I thought I'd clean it up slightly and make it an official feature.

I considered wrapping the SpectorJS-code in pragmas.debug, but it's pretty low impact so I decided against it in the end.

Fixes #5279

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

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

[kring]

By the way, requestRenderMode makes it slightly awkward to use the editor (you have to force Cesium to render again before the change is shown), so best to turn it off when you're editing shaders.

[mramato]

While we don't do a good job of it everywhere, in general this should be typeof spector !== 'undefined' instead using window, since window doesn't exist in all contexts (workers/node/etc..) it's a more robust check for a global spector variable

[kring]

Done!

[cesium-concierge]

Thanks again for your contribution @kring!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

[lilleyse]

Works perfectly for me. I confirmed that shader changes work and errors get propagated correctly.