Fixed Exponential Decay Calculation to Use Last Value in Window

Fixed point of origin for exponential decay window functions to the last value in window. Previously, decay was calculated by formula exp((t - curr_row_t) / decay_length), which is incorrect when right boundary of window is not CURRENT ROW. It was changed to: exp((t - last_row_t) / decay_length). There is no change in results for windows with ROWS BETWEEN (smth) AND CURRENT ROW. #39593 (Vladimir Chebotaryov).