Fix support for BGRA formats in the texture environment.

Bug 21766174

Change-Id: If2fbb543303db27a2a2bd13e1a75cb7f7310d6eb
Reviewed-on: https://swiftshader-review.googlesource.com/3478
Reviewed-by: Greg Hartman <ghartman@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>