libswscale/options: Add parent_log_context_offset to AVClass

This allows to associate log messages from slice contexts to
the user-visible SwsContext.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libswscale/options.c

@@ -91,6 +91,7 @@ const AVClass ff_sws_context_class = {
     .class_name = "SWScaler",
     .item_name  = sws_context_to_name,
     .option     = swscale_options,
+    .parent_log_context_offset = OFFSET(parent),
     .category   = AV_CLASS_CATEGORY_SWSCALER,
     .version    = LIBAVUTIL_VERSION_INT,
 };

libswscale/swscale_internal.h

@@ -301,6 +301,8 @@ typedef struct SwsContext {
      */
     const AVClass *av_class;
 
+    struct SwsContext *parent;
+
     AVSliceThread      *slicethread;
     struct SwsContext **slice_ctx;
     int                *slice_err;

libswscale/utils.c

@@ -1214,6 +1214,8 @@ static int context_init_threaded(SwsContext *c,
         if (!c->slice_ctx[i])
             return AVERROR(ENOMEM);
 
+        c->slice_ctx[i]->parent = c;
+
         ret = av_opt_copy((void*)c->slice_ctx[i], (void*)c);
         if (ret < 0)
             return ret;