glsl: only add outward facing varyings to resourse list for SSO

glsl: only add outward facing varyings to resourse list for SSO

An SSO program can have multiple stages and we only want to add the externally
facing varyings. The current code was adding both the packed inputs and outputs
for the first and last stage of each program.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>