Clarify GLES/WebGL difference with uninitialized compressed textures.

[kenrussell]

Fixes #3686.

[kenrussell]

@juj any feedback welcome. This seemed most appropriate to add to the WebGL 2.0 spec because this has always been disallowed in WebGL 1.0.

[lexaknyazev]

Using texStorage is not a functional replacement because it would prevent potential texture redefinitions. Assuming that we cannot retroactively allow null, the only option is to use new Uint8Array(n) with an appropriate size.

[kenrussell]

Fair point; I've rewritten this paragraph. Would you please take another look?

[juj]

LGTM, thanks for the update!