videodecoder: Limit QoS skip time

When video is running late, and qos handling is enabled, the decoder would skip at least 2*diff frames, where diff is how late frames are arriving downstream. If there's a timestamping bug or a clock glitch, this can lead to some very large amounts of video being skipped. Make the maximum length of frames that are skipped configurable, with a 0.5 second default - so we'll always output at least 1 frame and see if the downstream QoS problem has resolved.