avcodec: mark get_buffer thread safe

In libavcodec, thread-safe callback merely means callback that can run
on any thread. It does not imply that the codec should be re-entrant.
In other words, it boils down to not using thread-specific variables.

The VLC get_buffer callback is not reentrant currently, but it does not
care which thread it runs on.

modules/codec/avcodec/video.c

@@ -335,6 +335,7 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
     if( i_codec_id == AV_CODEC_ID_MPEG4 )
         p_sys->p_context->thread_count = 1;
 
+    p_sys->p_context->thread_safe_callbacks = true;
 #endif
 
     char *hw = var_CreateGetString( p_dec, "avcodec-hw" ); /* FIXME */