glsl: Refactor code for creating gl_PerVertex interface block.

glsl: Refactor code for creating gl_PerVertex interface block.

Currently, we create just a single gl_PerVertex interface block for
geometry shader inputs.  In later patches, we'll also need to create
an interface block for geometry and vertex shader outputs.  Moving the
code into its own class will make reuse easier.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>