glsl: add offset to glsl interface type

glsl: add offset to glsl interface type

In this patch we also copy the offset value from the ast and
implement offset linking rules by adding it to the record_compare()
function.

From Section 4.4.5 (Uniform and Shader Storage Block Layout Qualifiers)
of the GLSL 4.50 spec:

   "Two blocks linked together in the same program with the same block
   name must have the exact same set of members qualified with
   offset and their integral-constant-expression values must be the
   same, or a link-time error results."

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