Clarify getContext behavior with webgl / webgl2

As has been found in https://github.com/emscripten-core/emscripten/pull/13497 and https://github.com/emscripten-core/emscripten/issues/13295 , and pointed out by @kainino0x , the canvas getContext specification at:

https://html.spec.whatwg.org/multipage/canvas.html#dom-canvas-getcontext

is very unclear about whether the 'webgl' and 'webgl2' context types are compatible.

By design they should not be. WebGL2RenderingContext deliberately does not extend WebGLRenderingContext.

Chrome and Firefox implement the correct behavior; Safari does not. The conformance suite needs to be extended to cover this, and the canvas specification updated.

Start with the WHATWG canvas getContext specification and the WebGL conformance suite, using issues 13295 and pull request 13497 for context. Extend coverage for compatibility between the webgl and webgl2 context types and update the canvas specification so the intended behavior is explicit; done means the suite captures the expected distinction and the specification reflects it.

Written by the indexing model from the issue text.