video_output: keep track of the next picture when there is one

When we schedule a display of the current frame and it's not late, it will be scheduled to be displayed at VOUT_REDISPLAY_DELAY. But that will likely be too late to display the next picture.

So if we can tell when the next frame will need to be displayed, we should use that information instead.

Rename the swap and prepare dates, as "next" isn't really what the values are.

This partially reverts commit f1bf7ce5.