libobs: Save clamped video time

The "clamped" video time is the system time per video frame that is
closest to the current system time, but always divisible by the frame
interval.  For example, if the last frame system timestamp was 1600 and
the new frame is 2500, but the frame interval is 800, then the
"clamped" video time is 2400.

This clamped value is useful to get the relative system time without any
jitter.