lavc/videotoolboxdec: fix crop handling when multithreaded

This was partially fixed by 233cd890, but it made changes to AVFrame fields
from within end_frame, which doesn't work consistently when multithreading
is enabled. This is what the post_process function is for.
Signed-off-by: Aman Gupta <aman@tmm1.net>

libavcodec/videotoolbox.c

@@ -91,6 +91,11 @@ static int videotoolbox_postproc_frame(void *avctx, AVFrame *frame)
         return AVERROR_EXTERNAL;
     }
 
+    frame->crop_right = 0;
+    frame->crop_left = 0;
+    frame->crop_top = 0;
+    frame->crop_bottom = 0;
+
     frame->data[3] = (uint8_t*)ref->pixbuf;
 
     if (ref->hw_frames_ctx) {
@@ -898,11 +903,6 @@ static int videotoolbox_common_end_frame(AVCodecContext *avctx, AVFrame *frame)
     AVVideotoolboxContext *videotoolbox = videotoolbox_get_context(avctx);
     VTContext *vtctx = avctx->internal->hwaccel_priv_data;
 
-    frame->crop_right = 0;
-    frame->crop_left = 0;
-    frame->crop_top = 0;
-    frame->crop_bottom = 0;
-
     if (vtctx->reconfig_needed == true) {
         vtctx->reconfig_needed = false;
         av_log(avctx, AV_LOG_VERBOSE, "VideoToolbox decoder needs reconfig, restarting..\n");