Refactor sampling functions to use a return value.

Refactor sampling functions to use a return value.

Change-Id: Ib62f310abecbc4cdaf6e9300791600f25af0eaf3
Reviewed-on: https://swiftshader-review.googlesource.com/13550
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>