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/8bps.c

@@ -30,16 +30,13 @@
  *         : RGB32 (RGB 32bpp, 4th plane is alpha)
  */
 
-#include <stdio.h>
-#include <stdlib.h>
 #include <string.h>
 
+#include "libavutil/bswap.h"
 #include "libavutil/internal.h"
-#include "libavutil/intreadwrite.h"
 #include "avcodec.h"
 #include "codec_internal.h"
 #include "decode.h"
-#include "internal.h"
 
 
 static const enum AVPixelFormat pixfmt_rgb24[] = {