release v0.21.25

This fixes compatibility with Plex DLNA.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/851

Stop bothering people about the Tidal/Qobuz plugins.

Make LogError()/FormatError() wrappers for those.  Now we can log
exceptions with a lower level.

Prepare for templated functions.

Print all nested exceptions on a single line to avoid confusion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/864

Closes https://github.com/MusicPlayerDaemon/MPD/issues/841

Mounting one storage URI twice on different mount points can lead to
conflicts with the database cache file, and it doesn't make a lot of
sense.

But most importantly, our udisks storage plugin will unmount the disk
from the kernel VFS, and if two exist, they will compete with each
others.  We could (and should) fix this in the udisks storage plugin,
but for now, this workaround is good enough (and useful).

When mounting something over a directory that is already a mount
point, CompositeStorage::Mount() silently overwrites the previously
mounted storage, disposing it.  After that, SimpleDatabase::Mount()
will fail and handle_mount() will roll back the
CompositeStorage::Mount() command, effectively unmounting what was
there before (and also leaking memory).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/918

Fixes the "Unrecognized URI" error with the udisks storage plugin,
which is caused by the kludge in UdisksStorage::MapUTF8().

Fixes use-after-free crash bug during MPD shutdown.

Bug #915 is about an I/O exception thrown where none was allowed,
leading to crash via std::terminate().  However, instead of catching
and logging the error inside the decoder plugin, it should be able to
propagate the I/O error to the MPD core, so MPD can avoid trying other
decoder plugins.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/915

Wait until there is data in the ring buffer.

Wait until there is some data; don't let our render callback be
invoked without any data.

We shouldn't restart the AudioUnit while the ring buffer is empty, or
else our render callback may emit noise.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/771

This will keep track of AudioOutputUnitStart() and
AudioOutputUnitStop().  This will provide some separation between "not
(yet) (re)started" and "paused".

This shall be done by Close(), which will be called automatically
after an error.

Move build rules from src/output/plugins/meson.build