glsl: tidy up struct with a single member

glsl: tidy up struct with a single member

There used to be more members but they now share other fields
in order to keep memory use low.

Also making the naming more generic will allow us to reuse the
field for explicit byte offsets within blocks for
ARB_enhanced_layouts.

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>