qtdemux: fix key unit seek corner case
If a key unit seek is performed with a time position that matches the offset of a keyframe, but not its actual PTS, we need to adjust the segment nevertheless.
For example consider the following case:
- stream starts with a keyframe at 0 nanosecond, lasting 40 milliseconds
- user does a key unit seek at 20 milliseconds
- we don't adjust the segment as the time position is "over" a keyframe
- we push a segment that starts at 20 milliseconds
- we push a buffer with PTS == 0
- an element downstream (eg rtponviftimestamp) tries to calculate the stream time of the buffer, fails to do so and drops it