Since EAC is no longer decoded to an 8 bit format, but ETC2 alpha
still is, we need to distinguish between the 2. Added an argument
to DecodeBlock for that purpose.

Change-Id: Ied4263cecc43f88609a357437c11f4e43af229b8
Reviewed-on: https://swiftshader-review.googlesource.com/15228Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>