glsl: add process_qualifier_constant() helper

glsl: add process_qualifier_constant() helper

For now this just validates that a qualifier is inside its
minimum boundary, in a later patch we will expand it to
evaluate compile time constants.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>