GME Plugin: fix track numbering

GME starts all track indexes at zero, but subtune prefixes
start at one. This fixes an off-by-one error during track
enumeration.

NEWS

@@ -3,6 +3,7 @@ ver 0.20.11 (not yet released)
   - curl: support Content-Type application/xml
 * decoder
   - ffmpeg: more reliable song duration
+  - gme: fix track numbering
 * fix case insensitive search without libicu
 
 ver 0.20.10 (2017/08/24)

src/decoder/plugins/GmeDecoderPlugin.cxx

@@ -293,13 +293,13 @@ gme_container_scan(Path path_fs)
 	TagBuilder tag_builder;
 
 	auto tail = list.before_begin();
-	for (unsigned i = 1; i <= num_songs; ++i) {
+	for (unsigned i = 0; i < num_songs; ++i) {
 		ScanMusicEmu(emu, i,
 			     add_tag_handler, &tag_builder);
 
 		char track_name[64];
 		snprintf(track_name, sizeof(track_name),
-			 SUBTUNE_PREFIX "%03u.%s", i, subtune_suffix);
+			 SUBTUNE_PREFIX "%03u.%s", i+1, subtune_suffix);
 		tail = list.emplace_after(tail, track_name,
 					  tag_builder.Commit());
 	}