glsl: Keep track of centroid/interpolation mode for interface block members.

glsl: Keep track of centroid/interpolation mode for interface block members.

Fixes piglit tests:
- interface-block-interpolation-{array,named,unnamed}
- glsl-1.50-interface-block-centroid {array,named,unnamed}

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>