WIP MR: clock: add vlc_clock_leastsquares

Filter clock points with a least squares regression. The LSR computation can easily overflow since it sum squares of timestamps multiplied by update count. The overflow occurs after more than 10sec of normal playback. That is why we use several LSR contexts with a given time window (8sec):

t |----------------------------------------------------->
  |---8s---| First LSR (cls->index = 0)
     |---8s---| Second LSR (cls->index = 1)
        |---8s---| Third LSR (cls->index = 2)
           |---8s---| Fourth LSR (cls->index = 3)
              |---8s---| Fifth LSR (cls->index = 0)

One other advantage of using several contexts is that it will be more reactive to a sudden clock rate change.

Note that there is a safeguard if an overflow occurs within that small time window, the time window will be divided by 2 in that case. This case is very unlikely but could happen in case of burst of clock updates.

vlc_clock_leastsquares can be used when we know that the clock source is not precise.

• LSR vs moving average with clock test "lowprecision"

• LSR vs moving average with live rtsp