splitmuxsink: Override LATENCY query to pretend to downstream that we're not live

splitmuxsink can't possibly know how much latency it will introduce as it always keeps one GOP around before outputting something. This breaks the latency configuration of the pipeline and we're better off just pretending that everything downstream of the sinkpads is not live.

Especially muxers that are based on aggregator and time out on the latency deadline can easily misbehave otherwise as the deadline will be exceeded usually.