[util] Default to "text" output format in hb-shape

If you say:

  hb-shape font.ttf text --output-file out.txt

This was previously failing:

  Unknown output format `txt'; supported formats are: TEXT / JSON

Now we simply fallback to TEXT if no explicit format was requested.

util/hb-shape.cc

@@ -47,9 +47,14 @@ struct output_buffer_t
       output_format = hb_buffer_serialize_format_from_string (options.output_format, -1);
     if (!hb_buffer_serialize_format_to_string (output_format))
     {
-      fail (false, "Unknown output format `%s'; supported formats are: %s",
-	    options.output_format,
-	    g_strjoinv (" / ", (gchar**) hb_buffer_serialize_list_formats ()));
+      if (options.explicit_output_format)
+	fail (false, "Unknown output format `%s'; supported formats are: %s",
+	      options.output_format,
+	      g_strjoinv (" / ", (gchar**) hb_buffer_serialize_list_formats ()));
+      else
+	/* Just default to TEXT if not explicitly requested and the
+	 * file extension is not recognized. */
+	output_format = HB_BUFFER_SERIALIZE_FORMAT_TEXT;
     }
 
     unsigned int flags = HB_BUFFER_SERIALIZE_FLAGS_DEFAULT;

util/options.hh

@@ -313,6 +313,7 @@ struct output_options_t : option_group_t
   output_options_t (option_parser_t *parser) {
     output_file = NULL;
     output_format = NULL;
+    explicit_output_format = false;
 
     fp = NULL;
 
@@ -327,6 +328,9 @@ struct output_options_t : option_group_t
 
   void post_parse (GError **error G_GNUC_UNUSED)
   {
+    if (output_format)
+      explicit_output_format = true;
+
     if (output_file && !output_format) {
       output_format = strrchr (output_file, '.');
       if (output_format)
@@ -341,6 +345,7 @@ struct output_options_t : option_group_t
 
   const char *output_file;
   const char *output_format;
+  bool explicit_output_format;
 
   mutable FILE *fp;
 };