hlsdemux: fix reverse playback seek with snap flags

hlsdemux: fix reverse playback seek with snap flags

Properly handle snap flags during reverse seeking. In this case
the before/after are also reversed, so handle those as such.

For example: with a sequence of 1s fragments:

|-- 0 --|-- 1 --|-- 2 --|-- 3 --|

If you seek to 1.5s it is inside fragment 1. With reverse and

snap-before: should play from the end of fragment 1
snap-after: should play from the end of fragment 0