Add "av_stream_get_first_dts" symbol for www/qt5-webengine

Obtained from:

https://github.com/archlinux/svntogit-packages/blob/packages/ffmpeg/trunk/add-av_stream_get_first_dts-for-chromium.patch

--- libavformat/avformat.h.orig	2023-02-27 20:43:45 UTC
+++ libavformat/avformat.h
@@ -1019,6 +1019,10 @@ int64_t    av_stream_get_end_pts(const AVStream *st);
 int64_t    av_stream_get_end_pts(const AVStream *st);
 #endif
 
+// Chromium: We use the internal field first_dts vvv
+int64_t    av_stream_get_first_dts(const AVStream *st);
+// Chromium: We use the internal field first_dts ^^^
+
 #define AV_PROGRAM_RUNNING 1
 
 /**
--- libavformat/mux_utils.c.orig	2023-02-27 20:43:45 UTC
+++ libavformat/mux_utils.c
@@ -40,6 +40,13 @@ int64_t av_stream_get_end_pts(const AVStream *st)
 }
 #endif
 
+// Chromium: We use the internal field first_dts vvv
+int64_t av_stream_get_first_dts(const AVStream *st)
+{
+  return cffstream(st)->first_dts;
+}
+// Chromium: We use the internal field first_dts ^^^
+
 int avformat_query_codec(const AVOutputFormat *ofmt, enum AVCodecID codec_id,
                          int std_compliance)
 {