Reload playlist only when there is only 1 or 0 segments left that are
downloaded but not played yet. Also change the default interval to be
segment length instead of half segment length.

With this patch it downloads 3 segments from 5 seglen playlist at the
time and then plays those. Should be much nicer on http-server and
powersaving (marginally as there isn't network activity that often).

httplive: check segment gap on last segment number instead of the segment number of last playlist load

Bug fix: HLS module does not block until data is available. As a result live streams may end unexpectedly in case of short playlists or slow connections. Problem was fixed by blocking on a condition variable in the "read" function in case data is not available until Download thread signals (when new data is available) or a timeout of 10 seconds is reached. Blocking is done with a timed wait in order to avoid deadlocks since the thread that calls read is also responsible for calling close.
Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>

In case of live streams, the reload thread did not wake up the download thread and so new segments were not downloaded. Problem was fixed by sginalling the download thread with the condition variable.
Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>

This is required when building libpng.

More broken avi files :/

Close #9227

Some muxes, in particular mkv/webm, behave very differently depending on
whether we say that the stream is seekable or not (by providing the IOSeek
function). It does not help that the seek function itself returns an error.

Thus, add a new access_out control called ACCESS_OUT_CAN_SEEK, set to true
for seekable files in the file output only, and propagate the status of that
into avformat at initialization time.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>