release v0.20.13

Signed-off-by: FlashSystems <developer@flashsystems.de>

If `SimpleDatabase::Visit` is called on a database that contains a mounted directry the URIs of the elements passed to the callbacks are not prefixed by the mountpoint path. This leads to lsinfo and add not working because they use the wrong URI. This pull request is using the `WalkMount` helper function to create prefixed versions of `VisitDirectory`, `VisitSong` and `VisitPlaylist` to add the correct prefix to the parameters of the callback functions.

Closes #172

This alleviates a problem in which 'volume +1' cannot be undo by
'volume -1' when using alsa hw mixer.

Closes #104

[mk: the following text was copied from
https://github.com/MusicPlayerDaemon/MPD/pull/167]

For certain format (hi-res files) and normal buffer size hardware, The
hardware may at once consume most of the buffers. However, in Delay()
function, MPD is supposed to wait for 25 ms after the next try. it
will create a hiccup. The negative impact is much major than
increasing the latency.

I understand larger buffers come at a price. That's why in my earlier
commit last year I significantly reduced it. However, the buffer size
in CoreAudio is set according to the hardware, which is super small
latency. For instance, the system audio of 2015 generation of macbook
pro has maximum buffer size of 4096 samples, which is just 0.09s for
44.1k framerate, or 0.04s for 96k frames --- . compare to the 0.5 sec
latency alsa plugin has, even if we quadruple it, it's still super
tiny.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

If libupnp is installed in a non-standard location we must rely on the
include path provided by $(pkg-config --cflags libupnp). Relative to the
path given from that command no prefix must be used to find the respective
files.

assignment of `mnt->mounted_database` in `SimpleDatabase::Mount`.

This fixes spurious replay gain logs when the player inserts silence
chunks, because those silence chunks had no replay gain attached,
resetting the ReplayGainFilter state, flipping it forth and back.

This attribute is not particularly important, but it was
uninitialized.

Fixes #165.

When starting playback with a specific song which does not have the
highest priority, the previous highest priority song was skipped
completely because its order was "swapped".  This commit changes to a
more expensive operation which inserts the selected song into the
order list.

This fixes a small part of #165

libupnp 1.6.24 added a few badly designed macros which break the MPD
build:

 https://sourceforge.net/p/pupnp/code/ci/8177a4195a4d4a5d6cb095eaf4ca5b65ac446381/

To work around this, we disable our emulation functions (from
714011c8) on this libupnp version.

Closes #163

That '!' accidently got lost in commit
75582d47

D'oh!

Closes #162

Signed-off-by: FlashSystems <developer@flashsystems.de>