avcodec/avcodec: Avoid codec_desc.h, codec_par.h inclusions

Instead, use forward declarations; and in order not to affect
any user include these headers for them, but not internally.
This has the advantage of removing implicit inclusions of these
headers from almost all files providing codecs.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/encode.c

@@ -30,6 +30,7 @@
 
 #include "avcodec.h"
 #include "avcodec_internal.h"
+#include "codec_desc.h"
 #include "codec_internal.h"
 #include "encode.h"
 #include "frame_thread_encoder.h"