hls: Return all packets from a previous variant before moving on to the next one

This serves as a safeguard; normally we want to use the dts
comparison to interleave packets from all active variants. If that
dts comparison for some reason doesn't work as intended, make sure
that all packets in all variants for a certain sequence number have
been returned before moving on to the next one.

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