decoder/mad: make "gapless_mp3_playback" a block option

Remove another dependency on the config/Global library.

src/config/Migrate.cxx

@@ -56,8 +56,17 @@ MigrateCurlProxyConfig(ConfigData &config) noexcept
 				 "proxy_password");
 }
 
+static void
+MigrateMadConfig(ConfigData &config) noexcept
+{
+	MigrateParamToBlockParam(config, ConfigOption::GAPLESS_MP3_PLAYBACK,
+				 ConfigBlockOption::DECODER, "plugin", "mad",
+				 "gapless");
+}
+
 void
 Migrate(ConfigData &config) noexcept
 {
 	MigrateCurlProxyConfig(config);
+	MigrateMadConfig(config);
 }

src/config/Templates.cxx

@@ -68,7 +68,7 @@ const ConfigTemplate config_param_templates[] = {
 	{ "id3v1_encoding", false, true },
 	{ "metadata_to_use" },
 	{ "save_absolute_paths_in_playlists" },
-	{ "gapless_mp3_playback" },
+	{ "gapless_mp3_playback", false, true },
 	{ "auto_update" },
 	{ "auto_update_depth" },
 	{ "despotify_user", false, true },

src/decoder/plugins/MadDecoderPlugin.cxx

@@ -21,7 +21,7 @@
 #include "MadDecoderPlugin.hxx"
 #include "../DecoderAPI.hxx"
 #include "input/InputStream.hxx"
-#include "config/Global.hxx"
+#include "config/Block.hxx"
 #include "tag/Id3Scan.hxx"
 #include "tag/Rva2.hxx"
 #include "tag/Handler.hxx"
@@ -107,10 +107,10 @@ mad_fixed_to_24_buffer(int32_t *dest, const struct mad_synth *synth,
 }
 
 static bool
-mp3_plugin_init(gcc_unused const ConfigBlock &block)
+mp3_plugin_init(const ConfigBlock &block)
 {
-	gapless_playback = config_get_bool(ConfigOption::GAPLESS_MP3_PLAYBACK,
-					   DEFAULT_GAPLESS_MP3_PLAYBACK);
+	gapless_playback = block.GetBlockValue("gapless",
+					       DEFAULT_GAPLESS_MP3_PLAYBACK);
 	return true;
 }