glsl: Relax combinations of layout qualifiers with other qualifiers.

glsl: Relax combinations of layout qualifiers with other qualifiers.

Previously we disallowed any combination of layout with interpolation,
invariant, or precise qualifiers. There is very little spec guidance on
exactly which combinations should be allowed, but with ARB_sso it's
useful to allow these qualifiers with rendezvous-by-location.

Since it's unclear exactly where the layout qualifier should appear when
combined with other qualifiers, we will allow it anywhere before the
auxiliary storage qualifier.

This allows enough flexibility for all examples I've seen, while keeping
the auxiliary-storage-qualifier / storage-qualifier pair together (as
they are a single qualifier in the spec prior to
ARB_shading_language_420pack)

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>