Fix support for all GL_EXT_read_format_bgra formats.

Change-Id: I9b3977959aea8bd2711ebd94e06c0e09376d4d6e
Reviewed-on: https://swiftshader-review.googlesource.com/4768
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>