glsl: remove unused varyings before packing them

glsl: remove unused varyings before packing them

Previously we would pack varyings before trying to remove them, this
relied on the packing pass not packing varyings with a location of -1
to avoid packing varyings that should be removed.
However this meant unused varyings with an explicit location would be
packed before they could be removed when we enable packing of them in a
later patch.

V2: fix regression in V1 removing unused varyings in multi-stage SSO,
fix regression with single stage programs.

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