video_output: avoid dropping special pictures

Don't drop the first picture and still pictures if they are late. They may be the only pictures available for a while and we need something to display.

Simplify the render_now flag: always true except when we use a new picture that has its own timing (unless it's the first, we want it right away).

This effectively duplicates some of the current usage of the picture_t::b_force flag, which shouldn't have to decide rendering policies.