Uint8ClampedArray can not be passed where Uint8Array is accepted

Gregg Tavares pointed out on the public_webgl list that Uint8ClampedArray can no longer be passed to e.g. texImage2D with type and format RGBA/UNSIGNED_BYTE:
https://www.khronos.org/webgl/public-mailing-list/archives/1603/msg00001.php

This used to work when Uint8ClampedArray derived from Uint8Array, but the type hierarchy was changed when typed arrays were folded in to the ECMAScript spec. A conformance test needs to be added and WebGL implementations updated.

Start with the reported texImage2D behavior and the ECMAScript typed-array hierarchy described in the issue. Add a conformance test covering Uint8ClampedArray with RGBA/UNSIGNED_BYTE, then use its result to verify the affected WebGL implementations.

Written by the indexing model from the issue text.