AVCodec.channel_layouts is deprecated and Clang (unlike GCC)
warns when setting this field in a codec definition.
Fortunately, Clang (unlike GCC) allows to use
FF_DISABLE_DEPRECATION_WARNINGS inside a definition (of an FFCodec),
so that one can create simple macros to set AVCodec.channel_layouts
that also suppress deprecation warnings for Clang.

(Notice that some of the codec definitions were already
inside FF_DISABLE/ENABLE_DEPRECATION_WARNINGS (that were not
guarded by FF_API_OLD_CHANNEL_LAYOUT); these have been removed.
Also notice that setting AVCodec.channel_layouts was not guarded
by FF_API_OLD_CHANNEL_LAYOUT either, so testing disabling it
it without removing all the codeblocks would not have worked.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>