video_output: split the code to get the next picture and the display+deadline call

All patches except the first and last one are just code shuffling with no functional/logical changes.

The last patch changes the behavior when the current frame is not late to be displayed. In that case we should do a normal rendering, rather than trigger a redisplay (80 ms) which will likely try to show that picture too late.

The first patch changes the vlc_tick_now() used to decide is the redisplay is too early or if we can wait some more (for a new picture that might arrive in the meantime).

This should help with the first frame that may be shown late. Ref. !2018 (merged) in particular the last WIP commit !2018 (41789e3c)