glsl_type: Simplify glsl_channel_type

glsl_type: Simplify glsl_channel_type

This is C++ so we can just poke at the fields of glsl_type if we wish
and calling get_instance is way easier and more reliable than handling
each instance separately.  While we're at it, we re-arrange the base
type labels to match the enum order and add 8-bit type support.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>