During move of this code from vo_gpu_next.c to video.c someone(TM) tried
to be smart and simplify the expression. The num_vsync includes error
compensation which can cause it to display +-1 vsync at the same rate.
We explicitly don't want to include this in "ideal" parameters.
Also num_vsyncs was already rounded so we produced off timings in
general.
Revert back to proper way of translating the time.
Fixes: 5e5a325