Review WebGL2 spec for fingerprinting issues

WebGL2 is available in FF38, but is off by default via a pref (webgl.enable-prototype-webgl2). I imagine it is likely it will be finalized by FF45, so we should review the spec by then.

Here's the spec: https://www.khronos.org/registry/webgl/specs/latest/2.0/

This URL also proved useful in the past: https://www.browserleaks.com/webgl

See also #16005 (moved) and #13022 (moved).

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