player: don't quit with --loop-file

Fixes #1009.

CC: @mpv-player/stable
video: minor simplification of the old framedrop code

No changes in functionality, other than being slightly more correct at
stream EOF.
video: exit early when nothing to do

These cases were probably confusing. Exit early, which makes it much
clearer what's going on. Should not change anything functionally.
video: move some code around
video: move some more code around

No functional changes.
video: don't run new frame processing on every iteration

This ran adjust_sync() on every playloop iteration, instead of every
newly decoded frame. It seems this was idempotent in the common case,
but the code was originally designed to be run once only, so restore
that.
video: reduce non-sense messages when playing coverart

Don't print PTS warnings by skipping the normal video path.