Polymorphic classes should have virtual destructors.

Change-Id: Ia3c8e48b43ff96df9b9db2e4742fabcbd13ee853
Reviewed-on: https://swiftshader-review.googlesource.com/2666Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>