The code to compute the final RGB565 conversion is now
generated once instead of being generated 4 times.

Change-Id: Id90ddd28d6ec4fb957d8093e6b9689462751002b
Reviewed-on: https://swiftshader-review.googlesource.com/10609Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>