glsl: Add methods to retrive a varying's name and streamId.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/link_varyings.h

@@ -112,6 +112,16 @@ public:
       return !this->next_buffer_separator && !this->skip_components;
    }
 
+   const char *name() const
+   {
+      return this->orig_name;
+   }
+
+   unsigned get_stream_id() const
+   {
+      return this->stream_id;
+   }
+
    /**
     * The total number of varying components taken up by this variable.  Only
     * valid if assign_location() has been called.