avcodec/videotoolbox: fix opaque_ref handling

This is a partial fix - to fix videotoolbox fully (with multithreaded
hwaccel decoding), much more work would be required.

Before this change, an assertion failure would trigger:

  $ ffmpeg -hwaccel videotoolbox -hwaccel_output_format videotoolbox_vld -i h264.ts -f null -y /dev/null
  Assertion (frame->private_ref && frame->private_ref->size == sizeof(FrameDecodeData)) || !(avctx->codec->capabilities & (1 << 1)) failed at libavcodec/decode.c:620
Signed-off-by: Aman Gupta <aman@tmm1.net>

libavcodec/videotoolbox.c

@@ -27,6 +27,7 @@
 #include "libavutil/avutil.h"
 #include "libavutil/hwcontext.h"
 #include "bytestream.h"
+#include "decode.h"
 #include "h264dec.h"
 #include "hevcdec.h"
 #include "mpegvideo.h"
@@ -70,6 +71,10 @@ static int videotoolbox_buffer_copy(VTContext *vtctx,
 
 int ff_videotoolbox_alloc_frame(AVCodecContext *avctx, AVFrame *frame)
 {
+    int ret = ff_attach_decode_data(frame);
+    if (ret < 0)
+        return ret;
+
     frame->width  = avctx->width;
     frame->height = avctx->height;
     frame->format = avctx->pix_fmt;