[color] Add HB_COLOR

Normally I would have added to_string / from_string like other types
have.  But since we don't use hb_color_t much, I'm not going to do that.
Although, if we did, we could use it in hb-view to parse colors...

src/hb-common.h

@@ -446,16 +446,20 @@ hb_variation_to_string (hb_variation_t *variation,
 
 /**
  * hb_color_t:
- * ARGB data type for holding color values.
+ *
+ * Data type for holding color values.
  *
  * Since: REPLACEME
  */
 typedef uint32_t hb_color_t;
 
-#define hb_color_get_alpha(color) (color & 0xFF)
-#define hb_color_get_red(color)   ((color >> 8) & 0xFF)
-#define hb_color_get_green(color) ((color >> 16) & 0xFF)
-#define hb_color_get_blue(color)  ((color >> 24) & 0xFF)
+#define HB_COLOR(b,g,r,a) ((hb_color_t) HB_TAG ((b),(g),(r),(a)))
+
+#define hb_color_get_alpha(color)	((color) & 0xFF)
+#define hb_color_get_red(color)		(((color) >> 8) & 0xFF)
+#define hb_color_get_green(color)	(((color) >> 16) & 0xFF)
+#define hb_color_get_blue(color)	(((color) >> 24) & 0xFF)
+
 
 HB_END_DECLS