video_output: remove wait misfire hazard margin

The time taken by the prepare is compound of measurements taken in actual preparation steps. It's made of the time needed to execute the static filters, the interactive filters, and the time needed for the display to prepare.

VOUT_MWAIT_TOLERANCE is the margin of error accepted when the scheduler wakes up the vout_control_Pop deadline late compared to the deadline we choosed, or wakes up the vlc_clock_Wait late. It's supposed to move the deadline back wherever relevant to display a frame, to wait a little less that in the ideal instantaneous or real time case.

It doesn't represent "time spent" so there's no need to account it in the duration of the prepare as a discriminator for late frames. In addition, it's hardcoded time not dependant on whether the media or the system running the video output and accounts for 1/4 of the time budget for rendering a frame with 60fps.

Regression from a5d85a5f.