avformat/hls: Set AVFMT_TS_DISCONT flag on HLS input format

There have been many reports over the years about problems when
taking an HLS stream as input to `ffmpeg` where there are timestamp
discontinuities present. This is explicitly supported in the
HLS spec (EXT-X-DISCONTINUITY) and often used for ad injection.

Various fixes and work-arounds have been proposed over the years,
but one step that seems obvious, even if it's not a complete fix,
is to mark the HLS input format as supporting discontinuities. This
will prevent timestamp fixup logic in ffmpeg.c kicking in that ends
up mangling the timestamps unnecessarily.

I've tested this out with an example provided by Joe Koberg early
last year, and it is sufficient to allow `ffmpeg` to download and
mux the stream correctly. Joe had briefly suggested that other
situations can still be handled incorrectly, but this seems like
a strict improvement.

Joe's example:

https://s3.amazonaws.com/playon-test-videos/discont_test_new/discont_test.m3u8

Reviewed-by: Steven Liu <lq@onvideo.cn>
Reviewed-by: Dennis Mungai <dmngaie@gmail.com>

libavformat/hls.c

@@ -2326,7 +2326,7 @@ AVInputFormat ff_hls_demuxer = {
     .long_name      = NULL_IF_CONFIG_SMALL("Apple HTTP Live Streaming"),
     .priv_class     = &hls_class,
     .priv_data_size = sizeof(HLSContext),
-    .flags          = AVFMT_NOGENSEARCH,
+    .flags          = AVFMT_NOGENSEARCH | AVFMT_TS_DISCONT,
     .read_probe     = hls_probe,
     .read_header    = hls_read_header,
     .read_packet    = hls_read_packet,