BGRA8 was missing from the list of valid internal formats

Change-Id: Id5c08267e04cefaf621edecb963d65a45d40e041
Reviewed-on: https://swiftshader-review.googlesource.com/16128Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>