pthread_frame: Propagate sw_pix_fmt across threads

This is required by the VP9 hwaccels (both DXVA2 and VAAPI) when
threads are enabled.
Tested-by: Hendrik Leppkes <h.leppkes@gmail.com>

libavcodec/pthread_frame.c

@@ -250,6 +250,7 @@ static int update_context_from_thread(AVCodecContext *dst, AVCodecContext *src,
         dst->width     = src->width;
         dst->height    = src->height;
         dst->pix_fmt   = src->pix_fmt;
+        dst->sw_pix_fmt = src->sw_pix_fmt;
 
         dst->coded_width  = src->coded_width;
         dst->coded_height = src->coded_height;