player: show "neutral" position markers for OSD bars

This commit implements them for volume and some video properties.
audio: add a "weak" gapless mode, and make it default

Basically, this allows gapless playback with similar files (including
the ordered chapter case), while still being robust in general.

The implementation is quite simplistic on purpose, in order to avoid
all the weird corner cases that can occur when creating the filter
chain. The consequence is that it might do not-gapless playback in
more cases when needed, but if that bothers you, you still can use
the normal gapless mode.

Just using "--gapless-audio" or "--gapless-audio=yes" selects the old
mode.
client API: disable LIRC input by default

Not only should using libmpv hog such global resources; it's also very
unlikely an application embedding mpv will ever want to make use of
this.
video: fix another cover art corner case

Playing a video and then an audio file with cover art kept displaying
the last frame of the video. This was because the hasframe flag was set,
perhaps due to redrawing the last video frame before the cover art image
is decoded.
player: hide some messages in verbose mode

This started getting annoying.