lavc: reorder AVCodecContext fields.

Put related fields together, move the important ones closer to the
beginning, try to eliminate holes.

libavcodec/pthread.c

@@ -465,7 +465,7 @@ static int update_context_from_user(AVCodecContext *dst, AVCodecContext *src)
     dst->slice_flags = src->slice_flags;
     dst->flags2      = src->flags2;
 
-    copy_fields(skip_loop_filter, bidir_refine);
+    copy_fields(skip_loop_filter, subtitle_header);
 
     dst->frame_number     = src->frame_number;
     dst->reordered_opaque = src->reordered_opaque;