video: use dts as fallback when determining pts by sorting

This makes the new code equivalent with the old one, which often passed
dts as pts. Also rename some variables to clear up things.
video: remove commented code
cosmetics: rename video/audio reset functions

These used the suffix _resync_stream, which is a bit misleading. Nothing
gets "resynchronized", they really just reset state.

(Some audio decoders actually used to "resync" by reading packets for
resuming playback, but that's not the case anymore.)

Also move the function in dec_video.c to the top of the file.
Move some code from player to audio/video reset functions
video: refactor PTS code, add fall back heuristic to DTS

Refactor the PTS handling code to make it cleaner, and to separate the
bits that use PTS sorting.

Add a heuristic to fall back to DTS if the PTS us non-monotonic. This
code is based on what FFmpeg/Libav use for ffplay/avplay and also
best_effort_timestamp (which is only in FFmpeg). Basically, this 1. just
uses the DTS if PTS is unset, and 2. ignores PTS entirely if PTS is non-
monotonic, but DTS is sorted.

The code is pretty much the same as in Libav [1]. I'm not sure if all of
it is really needed, or if it does more than what the paragraph above
mentions. But maybe it's fine to cargo-cult this.

This heuristic fixes playback of mpeg4 in ogm, which returns packets
with PTS==DTS, even though the PTS timestamps should follow codec
reordering. This is probably a libavformat demuxer bug, but good luck
trying to fix it.

The way vd_lavc.c returns the frame PTS and DTS to dec_video.c is a bit
inelegant, but maybe better than trying to mess the PTS back into the
decoder callback again.

[1] https://git.libav.org/?p=libav.git;a=blob;f=cmdutils.c;h=3f1c667075724c5cde69d840ed5ed7d992898334;hb=fa515c2088e1d082d45741bbd5c05e13b0500804#l1431
video: if PTS is missing, make something up using the framerate

Also get rid of the PTS check _after_ filters. This means if there's a
video filter which unsets PTS, no warning will be printed. But we assume
that all filters are well-behaved enough by now.
video: replace d_video->pts field, change PTS jump checks

The d_video->pts field was a bit strange. The code overwrote it multiple
times (on decoding, on filtering, then once again...), and it wasn't
really clear what purpose this field had exactly. Replace it with the
mpctx->video_next_pts field, which is relatively unambiguous.

Move the decreasing PTS check to dec_video.c. This means it acts on
decoder output, not on filter output. (Just like in the previous commit,
assume the filter chain is sane.) Drop the jitter vs. reset semantics;
the dec_video.c determined PTS never goes backwards, and demuxer
timestamps don't "jitter".
af_lavi: actually free the filter graph on uninit

This was a memory leak.

Also remove the AF_CONTROL_COMMAND_LINE code, which was inactive. (It's
never called if the new option parser is used.)