manpage: correct the --mf options

The MPlayer style syntax ("-mf fps=10:type=png") was removed a while
ago, and now only the flat variants ("--mf-fps=10" etc.) work.

CC: @mpv-player/stable
demux: asynchronous seeking

This tells the demuxer thread that it should seek, instead of waiting
until the demuxer thread is ready.

Care has to be taken about the state between seek request and actual
seeking: newly demuxed packets have to be discarded. We can't just
flush when doing the actual seek, because the user thread could read
these packets.

I'm wondering if this could lead to issues due to relaxed ordering of
operations. But it should be fine, since seeking influences packet
reading only, and seeking is always strictly done before that.

Currently, this will have no advantages; unless audio is disabled. Then
seeking as well as normal playback can be non-blocking.
demux: fix timestamp type for seek calls

mpv/mplayer2/MPlayer use double for timestamps, but the demuxer API used
float.
audio: remove unused metadata field

This was used for replaygain at some point, until replaygain info was
passed through explicitly.
ad_lavc: drop questionable fallback code

If the decoder didn't set a samplerate, it was initialized from the
container samplerate.

This probably didn't make much sense, because it's passed to the
decoder on initialization (so it could definitely use it). It's an
artifact from commit 66a9eb57 (which removed some Matroska-specific non-
sense), and I've never seen it actually happen since it was made into a
warning. Just get rid of it.
audio: move initial decode to generic code

This commit mainly moves the initial decoding of data (done to probe the
audio format) to generic code. This will make it easier to make audio
decoding non-blocking in a later commit.

This commit also changes how decoders return data: instead of having
them write the data into a prepared buffer, they return a reference to
an internal buffer (by setting dec_audio.decoded). This makes it
significantly easier to handle audio format changes, since the decoders
don't really need to care anymore.
player: don't sleep after seeks
osd: properly handle OSD bar timeout

This could just remain stuck on the screen, until the playloop happened
to be run again.