8-bit textures are not precise enough but no tests were failing because
dEQP was using 4-bit backbuffers.

Preserve the old decode-to-8-bits paths because they are still used by
Chrome.

BUG=angleproject:2336

Change-Id: Ieb651325e2a05c85bcc97f8e6d868afaf37aff0d
Reviewed-on: https://chromium-review.googlesource.com/899701
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>