Allow quotation marks around feature tag in hb_feature_from_string()

With this, I believe we accept CSS feature strings completely.

src/hb-shape.cc

@@ -117,16 +117,34 @@ parse_feature_tag (const char **pp, const char *end, hb_feature_t *feature)
 {
   parse_space (pp, end);
 
-  const char *p = *pp;
-  char c;
+  char quote = 0;
+
+  if (*pp < end && (**pp == '\'' || **pp == '"'))
+  {
+    quote = **pp;
+    (*pp)++;
+  }
 
-  while (*pp < end && (c = **pp, ISALNUM(c)))
+  const char *p = *pp;
+  while (*pp < end && ISALNUM(**pp))
     (*pp)++;
 
   if (p == *pp || *pp - p > 4)
     return false;
 
   feature->tag = hb_tag_from_string (p, *pp - p);
+
+  if (quote)
+  {
+    /* CSS expects exactly four bytes.  And we only allow quotations for
+     * CSS compatibility.  So, enforce the length. */
+     if (*pp - p != 4)
+       return false;
+    if (*pp == end || **pp != quote)
+      return false;
+    (*pp)++;
+  }
+
   return true;
 }