videoencoder: Adjust output segment time when first buffer arrives

The videoencoder uses an output segment time that has not yet been adjusted when handling the first buffer, so a problem may occur depending on the adjustment time of the encoder.

For example, if the encoder sets a large value to adjustment time, the first running time used to determines forcing key unit is calculated to very large value, not 0.

Therefore, the running time of force-key-unit event is compared with the invalid running time of first frame, so the unexpected buffer can be the key unit.