avcodec/internal: Move ff_get_buffer() to decode.h

Only used by decoders (encoders have ff_encode_alloc_frame()).

Also clean up the other headers a bit while removing now redundant
internal.h inclusions.

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

libavcodec/smacker.c

@@ -28,8 +28,7 @@
  * Based on http://wiki.multimedia.cx/index.php?title=Smacker
  */
 
-#include <stdio.h>
-#include <stdlib.h>
+#include <stddef.h>
 
 #include "libavutil/channel_layout.h"
 
@@ -53,8 +52,6 @@
 #include "codec_internal.h"
 #include "decode.h"
 #include "get_bits.h"
-#include "internal.h"
-#include "mathops.h"
 
 typedef struct SmackVContext {
     AVCodecContext *avctx;