glsl: Validate aux storage qualifier combination with other qualifiers.

glsl: Validate aux storage qualifier combination with other qualifiers.

We've been allowing `centroid` and `sample` in all kinds of weird places
where they're not valid.

Insist that `sample` is combined with `in` or `out`;
and that `centroid` is combined with `in`, `out`, or the deprecated
`varying`.

V2: Validate this in a more sensible place. This does require an extra
case for uniform blocks members and struct members, though, since they
don't go through the normal path.

V3: Improve error message wording; eliminate redundant error generation
for inputs in VS or outputs in FS.

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