decoder/vorbis: faster tag scanning with ov_test_callback()

using ov_test_callback with function CALLBACKS_STREAMONLY will cause
scanning to stop after the comment field.  ov_open (and ov_test)
default to CALLBACKS_DEFAULT which scans the file structure causing a
huge slowdown.  The speed improvement is huge: It scanned my files
around 10x faster This procedure has been recommended by monthy (main
vorbis developer) and was said to be safe for scanning files.

NEWS

@@ -1,4 +1,6 @@
 ver 0.15.3 (2009/??/??)
+* decoders:
+  - vorbis: faster tag scanning with ov_test_callback()
 * output:
   - fix stuttering due to uninitialized variable
 * update: don't re-read unchanged container files

src/decoder/vorbis_plugin.c

@@ -383,7 +383,7 @@ vorbis_tag_dup(const char *file)
 		return NULL;
 	}
 
-	if (ov_open(fp, &vf, NULL, 0) < 0) {
+	if (ov_test_callbacks(fp, &vf, NULL, 0, OV_CALLBACKS_STREAMONLY) < 0) {
 		fclose(fp);
 		return NULL;
 	}