glsl: split default in layout qualifier merge

glsl: split default in layout qualifier merge

Currently, the default in layout qualifier merge performs specific
validation and merge.

We want to split out the validation from the merge so they can be done
independently.

Additionally, for simplification, the direction of the validation and
merge is changed so the ast_type_qualifier calling the method is the
one validated and merged against the default in qualifier.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>