Fix XRGB surface to RGB texture binding.

Change-Id: I6a1dd24d997c64596dc096893338b7f471e64896
Reviewed-on: https://swiftshader-review.googlesource.com/4516
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>