screen-cast/monitor-src: Record frames with presentation time

Pass the timestamp of the frame as the target timestamp of the
record. This makes the rudimentary frame throttling mechanism
inside MetaScreenCastStreamSrc work with the timing variability
that dynamic dispatch times introduced.
22 jobs for !2804 with gbsneto/screencast-frame-timing in 40 minutes and 26 seconds (queued for 1 second)
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