demux: fix video with demux_mpg (DVD playback)

Commit 4d016a9 changed how demuxers report the codec of each stream.
Some of that was missed in video.c, which is important for legacy
demuxers (demux_mpg was broken by this, which is needed for DVD
playback).

Not sure about the ASF/AVI related change, but this is also a legacy
demuxers only codepath.
mp_common: silence warning

int64_t was accidentally used with "%lld" format specifiers, which is
incorrect (even though long long int is always 64 bits, the type behind
int64_t can be different, e.g. it can be long int on 64 bit platforms).
Prefix keycode defines with MP_

Do this to reduce conflicts with <linux/input.h>, which contains some
conflicting defines.

This changes the meaning of MP_KEY_DOWN:

  KEY_DOWN is renamed to MP_KEY_DOWN (cursor down key)
  MP_KEY_DOWN is renamed to MP_KEY_STATE_DOWN (modifier for key down state)