glsl: add builtins for geometry shaders.

glsl: add builtins for geometry shaders.

v2 (Paul Berry <stereotype441@gmail.com>): Account for rework of
builtin_variables.cpp.  Use INTERP_QUALIFIER_FLAT for gl_PrimitiveID
so that it will obey provoking vertex conventions.  Convert to GLSL
1.50 style geometry shaders.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
v3 (Paul Berry <stereotype441@gmail.com>): Be less obscure about
setting interpolation field of gl_Primitive variables.

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