obs-ffmpeg: Use av_opt_set on context instead of priv_data

Certain ffmpeg parameters such as "bufsize" or "maxrate" have to be
applied to the context rather than to "priv_data". In order to make sure
options are still passed to the encoder setting set AV_OPT_SEARCH_CHILDREN.

plugins/obs-ffmpeg/obs-ffmpeg-output.c

@@ -183,7 +183,8 @@ static bool parse_params(AVCodecContext *context, char **opts)
 			*assign = 0;
 			value = assign + 1;
 
-			if (av_opt_set(context->priv_data, name, value, 0)) {
+			if (av_opt_set(context, name, value,
+				       AV_OPT_SEARCH_CHILDREN)) {
 				blog(LOG_WARNING, "Failed to set %s=%s", name,
 				     value);
 				ret = false;