av_log: show ffmpeg "info" messages with -v only

They are rarely useful in my opinion.

This commit was mainly motivated by this message:

    Video uses a non-standard and wasteful way to store B-frames ('packed B-frames'). Consider using a tool like VirtualDub or avidemux to fix it.

It's what's left over from the "Invalid and inefficient vfw-avi..."
warning that used to be printed when playing avi/divx files. Although
the new message is much better, it's still rather useless and poses
more questions than it answers. Besides, nobody wants to remux a file
when playing it, especially not if playback appears to be completely
fine. (There are some claims that these files raise CPU usage, but even
my old crappy CPU can decode low res avi/divx files at real time at
about x35 playback speed.)
mplayer: don't run heartbeat command while paused

This is commonly used to disable the screensaver with broken/non-
standard X screensavers. During pause, the screensaver should not be
disabled, so not calling this command while paused seems sensible.

See github issue #236.
mp_common: add function which accepts a format string to format playback time
screenshot: make it possible to format playback time in various ways
mplayer: attempt to skip playlist entries which can't be played

This is for situations when repeated attempts at playing a playlist
entry failed, and playlist navigation becomes impossible due to that.
For example, it wasn't possible to skip backwards past an unplayable
playlist entry:

    mpv file1.mkv doesntexist.mkv file3.mkv

You couldn't skip back to file1.mkv from file3.mkv. When running a
single "playlist_prev" command, doesntexist.mkv would be played, which
would fail to load. As reaction to the failure to load it, the next file
would be played, which is file3.mkv.

To make this even worse, the file could successfully load, but run only
for a split second. So just loading successfully isn't good enough.

Attempt to solve this by marking problematic playlist entries as failed,
and by having playlist_prev skip past such playlist entries. We define
failure as not being able to play more than 3 seconds (or failing to
initialize to begin with). (The 3 seconds are in real time, not file
duration.)

"playlist_prev force" still exhibits the old behavior.

Additionally, use the same mechanism to prevent pointless infinite
reloading if none of the files on the playlist exist. (See github issue

All in all, this is a heuristic, and later adjustments might be
necessary.

Note: forward skips (playlist_next) are not affected at all. (Except for
the interaction with --loop.)
gl_common: signal to GL backend whether we are probing

This is supposed to reduce the amount of useless error messages shown
during initialization of vo_opengl. If multiple backends are compiled,
usually only one of them will work. For example, on Linux both X and
Wayland backends can be compiled, but usually either Wayland or X is
running. Then, if Wayland is not running, but X is, trying to initialize
the Wayland backend should not spam the terminal with error messages.

Code using this should probably raise the log level for error messages
from MSGL_ERR to MSGL_V. Only error messages that are typically printed
in these situations should be changed. For example, if Wayland is
available, but some other part of initialization fails, the error
messages should probably be visible.
x11_common: probing

Not too sure about this. It sucks a bit that I had to add vo.probing.
wayland/common: probing
gl_common: add wayland backend before x11 backend
x11/common: print message according to probing status
wayland/common: print message according to probing status
Removed duplicating message-call in x11/common and wayland/common
wayland/common: use MP_MSG rather than mp_msg