Restore video RTP timestamps for RTSP streams

[joeblack2k]

Summary

• restore H264NALUnit timestamp fields with defensive zero-init
• propagate encoder timestamps onto outgoing video NAL units in VideoWorker
• use propagated video timestamps in IMPDeviceSource with fallback to gettimeofday() only when no encoder timestamp is available

Why

Direct RTSP from a Thingino camera on eufy_t8400x_t31x_sc3338_syn4343 was producing repeated timestamp errors in ffmpeg/Frigate:

• DTS discontinuity
• Application provided invalid, non monotonically increasing dts

A live binary-only deployment of this patch removed those warnings completely in direct source tests:

• before: 12 warnings in an 8-second direct RTSP probe
• after: 0 warnings in the same 8-second probe
• after: 0 warnings in a longer 20-second probe

Notes

• audio timestamp behavior is unchanged
• video still falls back to gettimeofday() if no propagated timestamp is present, so the change is low risk for streams that do not populate encoder timestamps