TagHandler: use a TagBuilder internally

Reduce heap allocator overhead.

src/decoder/FlacMetadata.cxx

@@ -23,6 +23,7 @@
 #include "tag/Tag.hxx"
 #include "tag/TagHandler.hxx"
 #include "tag/TagTable.hxx"
+#include "tag/TagBuilder.hxx"
 #include "replay_gain_info.h"
 
 #include <glib.h>
@@ -231,7 +232,9 @@ void
 flac_vorbis_comments_to_tag(Tag &tag,
 			    const FLAC__StreamMetadata_VorbisComment *comment)
 {
-	flac_scan_comments(comment, &add_tag_handler, &tag);
+	TagBuilder tag_builder;
+	flac_scan_comments(comment, &add_tag_handler, &tag_builder);
+	tag_builder.Commit(tag);
 }
 
 void

src/decoder/OpusDecoderPlugin.cxx

@@ -28,6 +28,7 @@
 #include "OggCodec.hxx"
 #include "CheckAudioFormat.hxx"
 #include "tag/TagHandler.hxx"
+#include "tag/TagBuilder.hxx"
 #include "InputStream.hxx"
 #include "util/Error.hxx"
 
@@ -222,14 +223,16 @@ MPDOpusDecoder::HandleBOS(const ogg_packet &packet)
 inline enum decoder_command
 MPDOpusDecoder::HandleTags(const ogg_packet &packet)
 {
-	Tag tag;
+	TagBuilder tag_builder;
 
 	enum decoder_command cmd;
 	if (ScanOpusTags(packet.packet, packet.bytes,
-			 &add_tag_handler, &tag) &&
-	    !tag.IsEmpty())
+			 &add_tag_handler, &tag_builder) &&
+	    !tag_builder.IsEmpty()) {
+		Tag tag;
+		tag_builder.Commit(tag);
 		cmd = decoder_tag(decoder, input_stream, std::move(tag));
-	else
+	} else
 		cmd = decoder_get_command(decoder);
 
 	return cmd;

src/decoder/VorbisComments.cxx

@@ -23,6 +23,7 @@
 #include "tag/Tag.hxx"
 #include "tag/TagTable.hxx"
 #include "tag/TagHandler.hxx"
+#include "tag/TagBuilder.hxx"
 #include "replay_gain_info.h"
 
 #include <glib.h>
@@ -138,13 +139,9 @@ vorbis_comments_scan(char **comments,
 Tag *
 vorbis_comments_to_tag(char **comments)
 {
-	Tag *tag = new Tag();
-	vorbis_comments_scan(comments, &add_tag_handler, tag);
-
-	if (tag->IsEmpty()) {
-		delete tag;
-		tag = NULL;
-	}
-
-	return tag;
+	TagBuilder tag_builder;
+	vorbis_comments_scan(comments, &add_tag_handler, &tag_builder);
+	return tag_builder.IsEmpty()
+		? nullptr
+		: tag_builder.Commit();
 }