glsl: Implement MESA_shader_integer_mix extension.

Because why doesn't GLSL allow you to do this already?

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

src/glsl/glcpp/glcpp-parse.y

@@ -1245,6 +1245,9 @@ glcpp_parser_create (const struct gl_extensions *extensions, int api)
 
 	      if (extensions->ARB_shading_language_420pack)
 	         add_builtin_define(parser, "GL_ARB_shading_language_420pack", 1);
+
+	      if (extensions->MESA_shader_integer_mix)
+	         add_builtin_define(parser, "GL_MESA_shader_integer_mix", 1);
 	   }
 	}