glsl: cross validate varyings with a component qualifier

glsl: cross validate varyings with a component qualifier

This change checks for component overlap, including handling overlap of
locations and components by doubles. Previously there was no validation
for assigning explicit locations to a location used by the second half
of a double.

V3: simplify handling of doubles and fix double component aliasing
detection

V2: fix component matching for matricies

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