Fix the recorded video start time

The event start time already includes the duration of the
pre_event frames. Do not back track the video by a further
pre_event_count.

Signed-off-by: Selva Nair <selva.nair@gmail.com>

src/zm_ffmpeg_camera.cpp

@@ -875,7 +875,7 @@ int FfmpegCamera::CaptureAndRecord(
 
           // Clear all packets that predate the moment when the recording began
           packetqueue->clear_unwanted_packets(
-              &recording, monitor->GetPreEventCount(), mVideoStreamId);
+              &recording, 0, mVideoStreamId);
 
           while ( (queued_packet = packetqueue->popPacket()) ) {
             AVPacket *avp = queued_packet->av_packet();