We were enumerating the color buffer formats by sized internal format,
which is simplified and made less fragile by retrieving the base
internal format and implementing the spec's validation tables more
directly.

Change-Id: I46d244b9ed738e927b0cb5fe6d1deebf6f0cbae5
Reviewed-on: https://swiftshader-review.googlesource.com/16988Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>