Add HB_FEATURE_GLOBAL_START/END

Fixes https://github.com/harfbuzz/harfbuzz/issues/1141

New API:
HB_FEATURE_GLOBAL_START
HB_FEATURE_GLOBAL_END

docs/harfbuzz-sections.txt

@@ -582,6 +582,8 @@ hb_set_union
 
 <SECTION>
 <FILE>hb-shape</FILE>
+HB_FEATURE_GLOBAL_END
+HB_FEATURE_GLOBAL_START
 hb_feature_t
 hb_feature_from_string
 hb_feature_to_string

src/hb-common.cc

@@ -877,8 +877,8 @@ parse_feature_indices (const char **pp, const char *end, hb_feature_t *feature)
 
   bool has_start;
 
-  feature->start = 0;
-  feature->end = (unsigned int) -1;
+  feature->start = HB_FEATURE_GLOBAL_START;
+  feature->end = HB_FEATURE_GLOBAL_END;
 
   if (!parse_char (pp, end, '['))
     return true;

src/hb-common.h

@@ -379,6 +379,19 @@ typedef void (*hb_destroy_func_t) (void *user_data);
 
 /* Font features and variations. */
 
+/**
+ * HB_FEATURE_GLOBAL_START
+ *
+ * Since: REPLACEME
+ */
+#define HB_FEATURE_GLOBAL_START	0
+/**
+ * HB_FEATURE_GLOBAL_END
+ *
+ * Since: REPLACEME
+ */
+#define HB_FEATURE_GLOBAL_END	((unsigned int) -1)
+
 typedef struct hb_feature_t {
   hb_tag_t      tag;
   uint32_t      value;