glsl: Add support for parsing [iu]samplerBuffer types in GLSL 1.40.

glsl: Add support for parsing [iu]samplerBuffer types in GLSL 1.40.

The samplerBuffer type will be undefined in !glsl 1.40, and the
keyword is marked as reserved.  The [iu]samplerBuffer types are not
marked as reserved pre-1.40, so they don't have separate tokens and
fall through to normal type handling.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>