demux: rename demux_packet.h to packet.h

This always bothered me.
audio/filter: rename af_tools.c to tools.c

This always bothered me.
ao_null: fix simulated buffer size

The size accidentally defaulted to 200 seconds instead of 200
milliseconds, which had fatal consequences when trying to use it.
vdpau_old: enable OpenGL interop

OpenGL interop was essentially disabled, because the decoder didn't
request vdpau device creation from vo_opengl.
player: deselect video track if initialization fails

This didn't have any consequences, other than suddenly reinitializing
video when it works again (such as with EDL timeline mixing video and
audio-only files).
player: select fallback stream in timeline code for better EDL handling

The intention of the existing code was trying to match demuxer-reported
stream IDs, instead of using possibly arbitrary ordering of the frontend
track list. But EDL files can consist of quite different files, for
which trying to match the stream IDs doesn't always make sense.
manpage: fix typo in --video-align-y description
timeline: add new EDL format

Edit Decision Lists (EDL) allow combining parts from multiple source
files into one virtual file. MPlayer had an EDL format (which sucked),
which mplayer2 tried to improve with its own format (which sucked). As
logic demands, mpv introduces its very own format (which sucks).

The new format should actually be much simpler and easier to use, and
its implementation is simpler and smaller too.
timeline: add edl:// URIs

Questionable change from user perspective, but internally needed to
implement the next commit. Also useful for testing timeline stuff.
player: add --merge-files option
timeline: remove support for the mplayer2 EDL format

It was a bit too complicated and inconvenient, and I doubt anyone
actively used it. The mpv EDL format should cover all use cases.