Fix implementation read format for RGB formats.

Bug 27690616

Change-Id: I28b62fc56b0b572c1c9e94499d8a0c4c56737f3c
Reviewed-on: https://swiftshader-review.googlesource.com/5190
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>