EXT_texture_rg adds support for these readback formats but validation was
not added.

BUG=angle:609

Change-Id: Iddd94bb8d8cf3b244a0d8a59cd4445ffa2ae61fe
Reviewed-on: https://chromium-review.googlesource.com/195176Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>