manpage: adjust y4m output example

The additional flags are needed to get the muxer to write the source
video frame rate (instead of a fallback) for 100% equivalence to the
replaced MPlayer/mplayer2 y4m writer.
mplayer: try to handle PTS forward jumps

Raw MPEG streams can contain PTS discontinuities. While the playback
core  has obvious code for handling PTS going backward, PTS going
forward was as far as I can see not handled.

This can be an issue with DVD playback. This wasn't caught earlier,
because DVD playback was just recently switched to demux_lavf, which
implies -no-correct-pts mode. This mode doesn't use PTS in the same way
as the normal playback mode, and as such was too primitive to be
affected by this issue.

Use the following heuristic to handle PTS forward jumps: if the PTS
difference between two frames is higher than 10 seconds, consider it a
reset. (Also, use MSGL_WARN for the PTS discontinuity warnings.)

In this particular case, the MPEG stream was going from pts=304510857
to pts=8589959849 according to ffprobe (raw timestamps), which seems a
bit strange.

Note that this heuristic breaks if the source video has unusually high
frame times. For example Rooster_Teeth_Podcast_191.m4a, an audio file
with a slide show encoded as MJPEG video track.
vo_lavc: silence warning when writing y4m

Apparently a ffmpeg issue. Hide the warning because it's annoying.
Workaround suggested by divVerent.
mplayer: save sub-visibility property
gl_video: remove redundant condition
options: remove --stereo

Whatever this was supposed to be originally, it doesn't have much value
anymore. It just forced ad_mpg123 to upmix mono to stereo by default
(the audio chain can do that). As an option, it was mostly useless and
misleading, so get rid of it.