lavf/rawdec: set avg_frame_rate

Timestamps in two FATE H.264 conformance tests now start at 1 instead
of 0, which also happens in some other H.264 tests before this commit
and so is not a big issue.
Conversely, timestamps in some HEVC conformance tests start from a
smaller value now.
Ideally this should be addressed later in a more general way.

h264-conformance-frext-frext2_panasonic_b no longer requires -vsync
passthrough.

libavformat/rawdec.c

@@ -86,6 +86,7 @@ int ff_raw_video_read_header(AVFormatContext *s)
     st->codecpar->codec_id = s->iformat->raw_codec_id;
     sti->need_parsing = AVSTREAM_PARSE_FULL_RAW;
 
+    st->avg_frame_rate = s1->framerate;
     sti->avctx->framerate = s1->framerate;
     avpriv_set_pts_info(st, 64, 1, 1200000);