core: add --deinterlace option, restore it with resume functionality

The --deinterlace option does on playback start what the "deinterlace"
property normally does at runtime. You could do this before by using the
--vf option or by messing with the vo_vdpau default options, but this
new option is supposed to be a "foolproof" way.

The main motivation for adding this is so that the deinterlace property
can be restored when using the video resume functionality
(quit_watch_later command).

Implementation-wise, this is a bit messy. The video chain is rebuilt in
mpcodecs_reconfig_vo(), where we don't have access to MPContext, so the
usual mechanism for enabling deinterlacing can't be used. Further,
mpcodecs_reconfig_vo() is called by the video decoder, which doesn't
have access to MPContext either. Moving this call to mplayer.c isn't
currently possible either (see below). So we just do this before frames
are filtered, which potentially means setting the deinterlacing every
frame. Fortunately, setting deinterlacing is stable and idempotent, so
this is hopefully not a problem. We also add a counter that is
incremented on each reconfig to reduce the amount of additional work per
frame to nearly zero.

The reason we can't move mpcodecs_reconfig_vo() to mplayer.c is because
of hardware decoding: we need to check whether the video chain works
before we decide that we can use hardware decoding. Changing it so that
this can be decided in advance without building a filter chain sounds
like a good idea and should be done, but we aren't there yet.
core: restore user-set video and audio filters with resume functionality

This requires adding a function that converts the filter list back to a
string.
vd_lavc: reset last_sample_aspect_ratio in uninit_avctx()

In init_vo(), if sh->aspect is 0 or last_sample_aspect_ratio is set,
sh->aspect is overwritten. With software decoding fallback behaviour,
this makes the aspect ratio from container ignored since
last_sample_aspect_ratio is already set in first try with hardware
decoding.
mplayer: change "Playing" message printed before playback start

Change it from "Playing file." to "Playing: file". The idea is that it
looks nicer without that trailing dot. (See github issue #229.)
mplayer: print a message when writing watch_later config for resume