decoder_api: emulate SEEK command for initial seek to CUE track

When playing a CUE track, the player thread waited for the decoder to
become ready, and then sent a SEEK command to the beginning of the CUE
track.  If that is near the start of the song file, and the track is
short enough, the decoder could have finished decoding already at that
point, and seeking fails.

This commit makes this initial seek more robust: instead of letting
the player thread deal with the difficult timings, let the decoder API
emulate a SEEK command, and return it to the decoder plugin, as soon
as the plugin finishes its initialization.

src/decoder_internal.h

@@ -36,6 +36,20 @@ struct decoder {
 	 */
 	double timestamp;
 
+	/**
+	 * Is the initial seek (to the start position of the sub-song)
+	 * pending, or has it been performed already?
+	 */
+	bool initial_seek_pending;
+
+	/**
+	 * Is the initial seek currently running?  During this time,
+	 * the decoder command is SEEK.  This flag is set by
+	 * decoder_get_virtual_command(), when the virtual SEEK
+	 * command is generated for the first time.
+	 */
+	bool initial_seek_running;
+
 	/**
 	 * This flag is set by decoder_seek_where(), and checked by
 	 * decoder_command_finished().  It is used to clean up after