BGRA support was removed in recent refactoring, since no dEQP
must pass test required it, but Chrome still requires it.

Change-Id: I638adce7dfeeaf85bb1f6d9889103fb4fde26d96
Reviewed-on: https://swiftshader-review.googlesource.com/16068Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>