demux: add functions to simplify demuxers

Some preparations to simplify demux_mkv and demux_lavf.

struct demux_stream manages state for each stream type that is being
demuxed (audio/video/sub). demux_stream is rather annoying, especially
the id and sh members, which are often used by the demuxers to determine
current stream and so on. Demuxers don't really have to access this,
except for testing whether a stream is selected and to add packets.

Add a new_sh_stream(), which allows creating streams without having the
caller specify any kind of stream ID. Demuxers should just use sh_stream
pointers, instead of multiple kinds of IDs and indexes.
demux_mkv: simplify use of demuxer API

mkv_track_t now references sh_stream directly, instead of using an ID.
Also remove all accesses to demux_stream (demuxer->video etc.).

Remove some slave-mode things on the way, like "ID_SID_..." messages.
demux_lavf: simplify

This removes the stream handling mess by using a single list for all
stream types.

One consequence is that new streams are always set to AVDISCARD_ALL,
which could be an issue if packets are read before initializing other
streams. However, this doesn't seem to an issue for various reasons,
so we don't do anything about it.

The new code strictly assumes that libavformat never removes or
reorders streams once added to AVFormatContext->streams. Undefined
behavior will result if it does.
command: try to switch subs too for program property

Untested, but why not.
demux: remove some unused things
demux_mkv: introduce new_demux_packet_from() and use it
demux: fix clearing of input padding

MP_INPUT_BUFFER_PADDING_SIZE and FF_INPUT_BUFFER_PADDING_SIZE are both
16. The doxygen for FF_INPUT_BUFFER_PADDING_SIZE says only the first 23
bits must to be 0, but this is probably a lie.