Wasapi output plugin won't start playing after being paused

The cause is that the scope guard in the WASAPI work thread
(WasapiOutputPlugin.cxx, function WasapiOutputThread::Work(), in the
while (true) loop) is set up too 'late' in the execution. There is one
condition ("if (data_in_frames >= buffer_size_in_frames)") when it is
hit, the loop will continue without executing the scope guard. This
scope guard is responsible for emptying the buffer again, and if the
buffer is not emptied, the above mentioned condition will stay true.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1451

ffmpeg from current git master no longer exposes
av_malloc() nor av_free() through other included
headers. directly include libavutil/mem.h to fix
compilation with (as-yet-unreleased) ffmpeg.

- supporting bmp and tiff seems outdated
- webp is more widely used for coverimages

This part of the AVIOContext API is not documented :-(

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1448

Must be initialized for the check in SetVolume().

The writing and reading of metadata involves lots of different programs
and libraries. Therefore it is prudent to point out how exactly MPD
receives metadata. Ideally this helps to point users to the right place
if their tags are not picked up correctly.

Don't return early from ScanOpusTags() if only
TagHandler::WantPicture() is set.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1413

lib/src/libmpdclient-2.19/meson.build:1:0: ERROR: Unknown options: "needs_exe_wrapper"

The "needs_exe_wrapper" option was incorrectly set under
[built-in options] rather than [properties].

Improves the changes made in 57687779 by
using AudioManager.ACTION_AUDIO_BECOMING_NOISY rather than listening for
wired headset unplug events or Bluetooth headset disconnect events. This
method is more flexible, allowing the feature to work on other types of
audio output device, as well as Bluetooth devices that don't set their
device class correctly. This change also has the benefit of being more
responsive, pausing the audio before it is rerouted to the built-in
speaker.

https://developer.android.com/guide/topics/media-apps/volume-and-earphones

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Fixes assertion failure when the submit queue is empty.