stream_cdda, demux_raw: always use s16le

stream_cdda's output format is linked to demux_raw's default audio
format, and at least we don't care enough to provide a separate
mechanism to let stream_cdda explicitly set the format, so they must
match.

Judging from the existing code, it looks like CDDA always outputs little
endian. stream_cdda.c changed this back to native endian (what demux_raw
expects). Just make them both little endian. This requires less code,
and also having a raw demuxer's behavior depend on the endianness of the
machine isn't very sane anyway.
Remove mpbswap.h

This was once central, but now it's almost unused. Only vf_divtc still
uses it for extremely weird and incomprehensible reasons. The use in
stream.c is trivial. Replace these, and remove mpbswap.h.
sub: approximate subtitle display in no-video mode

This makes subtitle display somewhat work if no video is displayed, but
a VO window exists (--force-window or cover art display).

The main problem with normal subtitle display is that it's locked to
video: it uses the video PTS as reference, and the subtitles advance
only if a new video frame is displayed. In audio-only mode on the other
hand, no video frame is ever displayed (or only 1 in the cover art
case). You would need a workaround to adjust the subtitle PTS, and you
would have to decide with what frequency to update the display. In
general, there is no "right" display FPS for subtitles. Some formats
(ASS) have animations parameterized by time, and any refresh rate could
be used.

Sidestep these problems by enabling the text OSD-based subtitle
mechanism. This is similar to --no-sub-ass, and updates and renders
subtitles with plain OSD. It has some caveats: no bitmap subs, somewhat
incorrect timing, no formatting. Timing in particular is a bit strange
and depends how often the audio output asks for new data, or other
events that happen to wakeup the playloop.
player: fix OSD redraw heuristic with audio-only mode

When using --force-window (and no video or cover art), this heuristic
prevents any redrawing during seeking. It should be applied only if
there is any form of video.
player: rate-limit OSD text update

There's no need to update OSD messages and the terminal status if nobody
is going to see it. Since the player doesn't block on video display
anymore, this update happens to often and probably burns slightly more
CPU than necessary. (OSD redrawing is handled separately, so it's just
mostly useless text processing and such.)

Change it so that it's updated only on every video frame or all 50ms
(whatever comes first).

For VO OSD, we could in theory try to lock to the OSD redraw heuristic
or the display refresh rate, but that's more complicated and doesn't
work for the terminal status.
player: move code to make playloop smaller

This is basically a cosmetic change, although it weirdly also affects
the percent position in encoding mode.
player: simplify OSD message handling code

Eliminate the remains of the OSD message stack. Another simplification
comes from the fact that we do not need to care about time going
backwards (we always use a monotonic time source, and wrapping time
values are practically impossible). What this code was pretty trivial,
and by now unnecessarily roundabout.

Merge get_osd_msg() into update_osd_msg(), and add_osd_msg() into
set_osd_msg_va().
player: do not wrongly clear OSD bar stops, reindent

set_osd_bar_chapters() always cleared the OSD bar stops, even if the
current bar was not the seek bar. Obviously it should leave the state of
the bar alone in this case.

Also change the function control flow so that we can drop one
indentation level, and do the equivalent change for the other OSD bar
functions.