Reenvision WebGL unit tests

[pjcozzi]

After #4281 (webgl-mock-for-tests branch), CI will be able to run all Cesium unit tests. Tests that require WebGL use an empty WebGL stub and skip readPixels and friends expectations - so all the code is ran, but not all of it is verified.

I want to move away from heavily using readPixels tests; those are integration tests with the Cesium renderer and WebGL implementation. Some will still be included in Cesium's test suite, but most tests relying on readPixels can be verified reasonably well with verifying only Cesium code, e.g., with spys, validating DrawCommands, etc. This will lead to a faster, easier to debug, more reliable, and more consistent test suite. Even if it is less precise, readPixels expectations rarely find issues in practice and will still be with the mix TBA.

• Update WebGL stubs for WebGL 2, WebGL 2 and WebGL Extensions Roadmap #797
• Consider removing WebGL test category
• Strategy for validating rendering code with less readPixels...and migration as reasonable.
• Tech blog post

Small cleanup

• Could clean up FramebufferSpec more using toRender by adding framebuffer, renderState, and epsilon to toRender

[pjcozzi]

Also check out https://github.com/evanw/webgl-recorder via @austinEng