Mark internal buffer variables private

src/hb-buffer.h

@@ -42,6 +42,8 @@ typedef struct _hb_glyph_info_t {
   hb_codepoint_t codepoint;
   hb_mask_t      mask;
   uint32_t       cluster;
+
+  /*< private >*/
   hb_var_int_t   var1;
   hb_var_int_t   var2;
 } hb_glyph_info_t;
@@ -51,6 +53,8 @@ typedef struct _hb_glyph_position_t {
   hb_position_t  y_advance;
   hb_position_t  x_offset;
   hb_position_t  y_offset;
+
+  /*< private >*/
   hb_var_int_t   var;
 } hb_glyph_position_t;