Another fix for iOS 12.2

The previous fix (adding `-copyts` to copy the timestamp) did only work until a certain uptime (about 8h) was reached.
Adding ``-timestamps abs`` forces the use of absolute (wallclock) time.
This does for some reason work instead of the (much smaller) timestamps after some hours of uptime...

CameraSource.js

@@ -203,7 +203,7 @@ Camera.prototype.handleStreamRequest = function (request) {
 
     this.log(`Starting video stream (${width}x${height}, ${fps} fps, ${bitrate} kbps)`)
     let ffmpegCommand = `\
--f video4linux2 -input_format h264 -video_size ${width}x${height} -framerate ${fps} -i /dev/video0 \
+-f video4linux2 -input_format h264 -video_size ${width}x${height} -framerate ${fps} -timestamps abs -i /dev/video0 \
 -vcodec copy -copyts -an -payload_type 99 -ssrc ${ssrc} -f rtp \
 -srtp_out_suite AES_CM_128_HMAC_SHA1_80 -srtp_out_params ${srtp} \
 srtp://${address}:${port}?rtcpport=${port}&localrtcpport=${port}&pkt_size=1378`