mplayer, vo_image: simplify OSD redrawing, fix vo_image crash

vo_image didn't handle OSD redrawing correctly anymore after OSD
redrawing behavior was changed in commit ed9295c (or maybe it has been a
problem for a longer time, and only showed up now). Basically, flip_page
was called unexpectedly and when no image was stored, which made it
crash trying to access the image. This could happen when for example
provoking OSD redrawing by pausing while using --vo=image, or by using
this command line: mpv --vo=image '-vf=lavfi="select=not(mod(n\,3))"'

Fix by removing the code that pretends vo_image can redraw OSD, and by
removing the framestepping fallback, which could make bad things happen
if the VO didn't support OSD redrawing. By now, there aren't any real
VOs that can't redraw the OSD properly, so this code is not needed and
just complicates things like vo_image.

This change likely will also be useful for vo_lavc (encoding).