f_decoder_wrapper: reorganize, fix EDL/ordered chapters backward playback

Before this commit, there was a single process_decoded_frame() function.
It handled various aspects of dealing with a newly decoded frame. Move
some of these to a separate process_output_frame() function.

This new function is called in the order the frames are returned to the
playback core. Some correct_audio_pts() (was process_audio_frame())
becomes slightly less awkward due to this, and the timestamp smoothing
can actually work in backward playback mode now (thus moving p->pts out
of reset_decoder()).

Behavior for normal playback also changes subtly. This shouldn't matter
in sane cases, but if you mix broken files, --no-correct-pts, and
timeline stuff, differences in behavior might be visible.

Timeline clipping (EDL/ordered chapters) works now, because it's done
before "transforming" the timestamps. Audio timestamp smoothing happens
after it, which is a behavior change, but should be more correct. This
still runs crazy_video_pts_stuff() before everything else. On the pther
hand, --no-correct-pts or missing timestamp processing is done last. But
these things didn't really work with timeline before.

DOCS/man/options.rst

@@ -464,8 +464,6 @@ Playback Control
       audio formats tend to have problems - there are hacks for dealing with
       them, which may or may not work.
 
-    - Function with EDL/mkv ordered chapters is obviously broken.
-
     - Backward demuxing of subtitles is not supported. Subtitle display still
       works for some external text subtitle formats. (These are fully read into
       memory, and only backward display is needed.) Text subtitles that are
@@ -473,7 +471,7 @@ Playback Control
       correctly.
 
     - Some features dealing with playback of broken or hard to deal with files
-      will be disabled (such as timestamp correction).
+      will not work fully (such as timestamp correction).
 
     - If demuxer low level seeks (i.e. seeking the actual demuxer instead of
       just within the demuxer cache) are performed by backward playback, the