+#define PROG_PARAM_BIT_CENTROID 0x1 /**< for varying vars (GLSL 1.20) */
+#define PROG_PARAM_BIT_INVARIANT 0x2 /**< for varying vars (GLSL 1.20) */
+#define PROG_PARAM_BIT_FLAT 0x4 /**< for varying vars (GLSL 1.30) */
+#define PROG_PARAM_BIT_LINEAR 0x8 /**< for varying vars (GLSL 1.30) */