squash! glsl: add builtins for EXT/ARB_geometry_shader4

Allow GL_{ARB,EXT}_geometry_shader4 extension to be specified in
fragment shaders, so that the gl_PrimitiveID fragment shader input is
accessible.
squash! glsl: add builtins for EXT/ARB_geometry_shader4

Use INTERP_QUALIFIER_FLAT for gl_PrimitiveID so that it will obey
provoking vertex conventions.