Add gapless support

Gapless as a seamless transition between 2 audio media, this MR is not about cross-fading (not planned for VLC 4.0).

If gapless is enabled and possible, the vlc_player_t will load a new input_thread_t, in gapless mode, near the end of the current input_thread_t.

This input_thead_t in gapless mode will act as a normal input_thread_t except that it will be halted at the end of the buffering, waiting for a signal from the player to resume. The gapless preparation can be cancelled (if the user stop/seek/etc...).

The audio output is also modified to handle gapless. If an aout stream is notified to have a next stream playing (input_thread_t in gapless mode), it won't drain and stop at the end of the current stream.

I fixed the points discussed during the last VLC tech meeting https://code.videolan.org/videolan/vlc/-/wikis/Meetingreports/2020-10-04-VLC-Tech-Meeting-via-videoconf#gapless

• Gapless is only enabled if the input can be paced (Use it only for file-based access: file, smb, nfs...)
• Gapless is only enabled when playing only one audio ES (that drive the block == master)
• It can only work if the reported input time and length are correct

include/vlc_input_item.h

@@ -427,7 +427,8 @@ typedef struct input_item_parser_cbs_t
      * @note This callback is optional.
      *
      * @param item the parsed item
-     * @param subtree sub items of the current item
+     * @param subtree valid sub items of the current item, to release with
+     * input_item_node_Delete()
      * @param userdata user data set by input_item_Parse()
      */
     void (*on_subtree_added)(input_item_t *item, input_item_node_t *subtree, void *userdata);