avcodec/internal: Move FF_CODEC_CAP_* to a new header codec_internal.h

Also move FF_CODEC_TAGS_END as well as struct AVCodecDefault.
This reduces the amount of files that have to include internal.h
(which comes with quite a lot of indirect inclusions), as e.g.
most encoders don't need it. It is furthemore in preparation
for moving the private part of AVCodec out of the public codec.h.

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

libavcodec/avcodec.c

@@ -34,6 +34,7 @@
 #include "libavutil/thread.h"
 #include "avcodec.h"
 #include "bsf.h"
+#include "codec_internal.h"
 #include "decode.h"
 #include "encode.h"
 #include "frame_thread_encoder.h"