decoder/Bridge: truncate last chunk at the exact end_time

Instead of passing whole chunks to the MusicPipe and checking the
end_time after each chunk, truncate the last chunk if it would exceed
the end_time.  This requires keeping track of the absolute PCM frame
number.

This fixes a problem with gapless CUE song transitions: a small part
of the following song was always played twice.

Closes #113

src/decoder/Bridge.hxx

@@ -49,6 +49,11 @@ public:
 	 */
 	double timestamp = 0;
 
+	/**
+	 * The time stamp of the next data chunk, in PCM frames.
+	 */
+	uint64_t absolute_frame = 0;
+
 	/**
 	 * Is the initial seek (to the start position of the sub-song)
 	 * pending, or has it been performed already?