qtdemux: fix reverse playback EOS conditions

qtdemux: fix reverse playback EOS conditions

In reverse playback, we don't want to rely on the position of the current
keyframe to decide a stream is EOS: the last GOP we push will start with
a keyframe, which position is likely to be outside of the segment.

Instead, let the normal seek_to_previous_keyframe mechanism do its job,
it works just fine.