DecoderAPI: add function decoder_read_full()

Move code from the "mad" plugin.

src/decoder/MadDecoderPlugin.cxx

@@ -353,18 +353,8 @@ MadDecoder::ParseId3(size_t tagsize, Tag **mpd_tag)
 		memcpy(allocated, stream.this_frame, count);
 		mad_stream_skip(&(stream), count);
 
-		while (count < tagsize) {
-			size_t len;
-
-			len = decoder_read(decoder, input_stream,
-					   allocated + count, tagsize - count);
-			if (len == 0)
-				break;
-			else
-				count += len;
-		}
-
-		if (count != tagsize) {
+		if (!decoder_read_full(decoder, input_stream,
+				       allocated + count, tagsize - count)) {
 			LogDebug(mad_domain, "error parsing ID3 tag");
 			g_free(allocated);
 			return;