hls: Store first_timestamp in units of AV_TIME_BASE

When first_timestamp was stored as-is, its actual time base
wasn't known later in the seek function.

Additionally, the logic (from 795d9594cfa) for scaling it
based on stream_index is flawed - stream_index in the seek
function only specifies which stream the seek timestamp refers
to, but obviously doesn't say anything about which stream
first_timestamp belongs to.

In the cases where stream_index was >= 0 and all streams had the
same time base, this didn't matter in practice.

Seeking taking first_timestamp into account is problematic
when one variant is mpegts (with real timestamps) and one variant
is raw ADTS (with timestamps only being accumulated packet
duration), where the variants start at totally different timestamps.

Signed-off-by: Martin Storsjö <martin@martin.st>
hls: Store all durations in AV_TIME_BASE

Also parse segment durations as floating point, which is allowed
since HLS version 3.

This is based on a patch by Zhang Rui.

Signed-off-by: Martin Storsjö <martin@martin.st>
hlsproto: Store all durations in AV_TIME_BASE

Also parse segment durations as floating point, which is allowed
since HLS version 3.

Signed-off-by: Martin Storsjö <martin@martin.st>
hls: Free packets when skipping packets when seeking

This fixes memory leaks present since 2b3d041cdc.

Signed-off-by: Martin Storsjö <martin@martin.st>