🩹 Use short VBV buffer for h.264 encoders

Use short VBV buffer length targeting 120 milliseconds. x264 defines vbv buffer length as milliseconds time while nvenc defines number of kilobits. Using 250 kilobits depth gives around 120 milliseconds for a 2048 kbit/s stream.

This technically lowers quality but make very fast recovery when we have frame loss caused by packet loss.