Eliminate global variables, convert them to MixerMemento fields.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1583

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1529

This PR provides forward and backward compatibility at macos SDK 12.0. At SDK 12.0, API function names were changed essentially replacing
occurrences of the word Master/master with Main/main. This change was test built on two different systems.

1. macos 10.15.7 with Xcode 12.4 and clang 12.0.0 on x86_64
2. macos 12.5 with Xcode 13.4.1 and clang 13.1.6 on arm64 (Apple silicon M1)

It should be noted that on macos 10.15.7 with Xcode 11.2 and clang 11.0, MPD will not build.
The MPD documentation states that clang 11.0 is the minimum requirement,
but clang 11.0 produces compile errors. Apparently the macos version
of clang 11.0 is not fully compliant.

Commit ebae25d1 added that #include, but forgot to wrap it in
"extern C", so the linker tried to look up C++ symbols, causing linker
failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1582

Properly deals with iconv, unlike the current solution. have_iconv fails
when libiconv CFLAGS are passed to the compiler. Tested under OpenWrt
with its CONFIG_BUILD_NLS, which adds libiconv include flags.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Closes #1564

Required by Google Play.

This is required for targetSdkVersion=30.

apksigner requires running zipalign first.

Deactivate the stream in Cancel().  This fixes stuttering after a
manual song change by refilling the whole ring buffer before
reactivating the stream.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1354

Clear not only MPD's ring buffer, but also libpipewire's buffers, to
avoid playing some audio from the previous song after a manual song
change.

Fixes part 1 of https://github.com/MusicPlayerDaemon/MPD/issues/1354

Removing the LockHasClients(); this code was copied from the "httpd"
output plugin, but unlike "httpd", the SnapCast output plugin does not
feed silence while paused, so we need to implement a delay to avoid
busy-looping the CPU.

As a side effect, this eliminates the suttering after resuming
playback, because the timer now gets reset even if there is a client.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1394

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1484

After connecting, PipeWire sometimes sends SPA_PROP_channelVolumes
with no values, and this led to "volume=-NaN".

Since PipeWire 0.3.53, there is no control name anymore, therefore the
name check doesn't work anymore, breaking volume change events.

This obsoletes the crash bug fix in commit 2ee57f9b

Fixes a potential crash bug.

This is a lower-level function without some of the clutter of
PipeWireOutput::SetVolume() which is not needed in that case.

This disables a long delay for playing songs from the cdio_paranoia
input plugin if ReplayGain is enabled.

The Read() method is not required to fill the whole buffer.  By
returning as soon as at least one byte was read, we allow faster
cancellation.

If seeking fails, don't leave the class with a wrong offset.