lavc: add a header for internal generic-layer APIs

The goal is to distinguish between APIs provided by the generic layer to
individual codecs and APIs internal to the generic layer.

Start by moving ff_{decode,encode}_receive_frame() and
ff_{decode,encode}_preinit() into this new header, as those functions
are called from generic code and should not be visible to individual
codecs.

libavcodec/avcodec.c

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