ffmpeg: pass the hwaccel frames context to the decoder

ffmpeg.c

@@ -2547,6 +2547,13 @@ static enum AVPixelFormat get_format(AVCodecContext *s, const enum AVPixelFormat
             }
             continue;
         }
+
+        if (ist->hw_frames_ctx) {
+            s->hw_frames_ctx = av_buffer_ref(ist->hw_frames_ctx);
+            if (!s->hw_frames_ctx)
+                return AV_PIX_FMT_NONE;
+        }
+
         ist->active_hwaccel_id = hwaccel->id;
         ist->hwaccel_pix_fmt   = *p;
         break;