glsl: simplifies the merge of the default in layout qualifier

glsl: simplifies the merge of the default in layout qualifier

The merge into the default in layout qualifier duplicates a lot of
code that can be reused from the generic merge method.

Now, we use the generic merge method inside the specific merge for the
default in layout qualifier. The generic merge method has been
completed with some bits that were only present in the merge for the
default in layout qualifier and the specific validation bits have been
moved to the validation method for the default in layout qualifier.

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