Define webgl2 context

[annevk]

@junov it seems webgl2 is not defined in the matrix we have? Probably best after #3370.

[kdashg]

It functions the same as webgl1 as far as canvas is concerned, just with a different id.

[kenrussell]

Sorry about that. I'll update the whatwg spec to add the "webgl2" context type -- already started. @annevk can you assign this to me?

[annevk]

Thanks @kenrussell! Note if you indeed already started you'll have to rebase as per OP.