[var] Add macros for standard axes

src/hb-ot-var.h

diff --git a/src/hb-ot-var.h b/src/hb-ot-var.h
index bce7c7a..ea69aa5 100644
--- a/src/hb-ot-var.h
+++ b/src/hb-ot-var.h
@@ -39,6 +39,12 @@
 #define HB_OT_TAG_fvar HB_TAG('f','v','a','r')
 #define HB_OT_TAG_avar HB_TAG('a','v','a','r')
 
+#define HB_OT_TAG_VAR_AXIS_ITALIC	HB_TAG('i','t','a','l')
+#define HB_OT_TAG_VAR_AXIS_OPTICAL_SIZE	HB_TAG('o','p','s','z')
+#define HB_OT_TAG_VAR_AXIS_SLANT	HB_TAG('s','l','n','t')
+#define HB_OT_TAG_VAR_AXIS_WIDTH	HB_TAG('w','d','t','h')
+#define HB_OT_TAG_VAR_AXIS_WEIGHT	HB_TAG('w','g','h','t')
+
 
 /*
  * fvar / avar