avutil/channel_layout: Add a new channel layout API

The new API is more extensible and allows for custom layouts.
More accurate information is exported, eg for decoders that do not
set a channel layout, lavc will not make one up for them.

Deprecate the old API working with just uint64_t bitmasks.

Expanded and completed by Vittorio Giovara <vittorio.giovara@gmail.com>
and James Almer <jamrial@gmail.com>.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/version.h

@@ -112,6 +112,7 @@
 #define FF_API_FIFO_PEEK2               (LIBAVUTIL_VERSION_MAJOR < 58)
 #define FF_API_FIFO_OLD_API             (LIBAVUTIL_VERSION_MAJOR < 58)
 #define FF_API_XVMC                     (LIBAVUTIL_VERSION_MAJOR < 58)
+#define FF_API_OLD_CHANNEL_LAYOUT       (LIBAVUTIL_VERSION_MAJOR < 58)
 
 /**
  * @}