ffv1dec: Avoid unnecessarily large stack usage and copies.

Ideally the compiler could figure this out on its own,
but it seems it can't.
An alternative that would avoid the messy explicit memcpy
would be to use a sub-struct for the parts that should
be preserved, which can then simply be assigned.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/ffv1dec.c

@@ -1051,12 +1051,17 @@ static int update_thread_context(AVCodecContext *dst, const AVCodecContext *src)
         return 0;
 
     {
-        FFV1Context bak = *fdst;
+        ThreadFrame picture = fdst->picture, last_picture = fdst->last_picture;
+        uint8_t (*initial_states[MAX_QUANT_TABLES])[32];
+        struct FFV1Context *slice_context[MAX_SLICES];
+        memcpy(initial_states, fdst->initial_states, sizeof(fdst->initial_states));
+        memcpy(slice_context,  fdst->slice_context , sizeof(fdst->slice_context));
+
         memcpy(fdst, fsrc, sizeof(*fdst));
-        memcpy(fdst->initial_states, bak.initial_states, sizeof(fdst->initial_states));
-        memcpy(fdst->slice_context,  bak.slice_context , sizeof(fdst->slice_context));
-        fdst->picture      = bak.picture;
-        fdst->last_picture = bak.last_picture;
+        memcpy(fdst->initial_states, initial_states, sizeof(fdst->initial_states));
+        memcpy(fdst->slice_context,  slice_context , sizeof(fdst->slice_context));
+        fdst->picture      = picture;
+        fdst->last_picture = last_picture;
         for (i = 0; i<fdst->num_h_slices * fdst->num_v_slices; i++) {
             FFV1Context *fssrc = fsrc->slice_context[i];
             FFV1Context *fsdst = fdst->slice_context[i];