decoder, player: support song ranges

Seek the decoder to the start of the range before beginning with
playback.  Stop the decoder when the end of the range has been
reached.  Add the start position to the seek position.  Expose the
duration of the range, not the full song file.

src/decoder_api.c

@@ -328,8 +328,9 @@ decoder_data(struct decoder *decoder,
 		}
 
 		dest = music_chunk_write(chunk, &dc->out_audio_format,
-					 decoder->timestamp, kbit_rate,
-					 &nbytes);
+					 decoder->timestamp -
+					 dc->song->start_ms / 1000.0,
+					 kbit_rate, &nbytes);
 		if (dest == NULL) {
 			/* the chunk is full, flush it */
 			decoder_flush_chunk(decoder);
@@ -366,6 +367,12 @@ decoder_data(struct decoder *decoder,
 
 		decoder->timestamp += (double)nbytes /
 			audio_format_time_to_size(&dc->in_audio_format);
+
+		if (dc->song->end_ms > 0 &&
+		    decoder->timestamp >= dc->song->end_ms / 1000.0)
+			/* the end of this range has been reached:
+			   stop decoding */
+			return DECODE_COMMAND_STOP;
 	}
 
 	return DECODE_COMMAND_NONE;