Oh the horror!  This plugin cannot possibly ever have worked.  It was
broken from the start, when it was added in commit 37796699 nearly
twelve (!) years ago.

The plugin would always read at sector boundaries, so it could only
ever work at multiples of 2 kB.

This makes the unit test fail.  D'oh!

MPD uses soxr with prefined resample recipes. Soxr also support defining a recipe your self.
This commit will support a custom recipe by changing the existing quality setting to "custom".

The same structs as the predefined recipes uses can now set by hand.

This will make the following settings available:
- precision 16|20|24|28|32 bits, example "28"
- phase_response - 0-100, example "45"
- passband_end - used bandwidth of source 80-99.7%, example "99.7.0"
- stopband_begin - anti aliasing 100.0+%, example "100".
- attenuation - signal reduciton in dB's, 0-30. example "3.0".
- flags "0" - additional bitmask with extra settings

The data is set in the structs soxr_quality_spec and soxr_io_spec (found in soxr.h).

Don't silently return 0 when there is no more data, because this may
crash the caller.  And flush output even if input EOF has been reached.

Sometimes, zzip_file_read() returns 0 even though the end of the file
was not reached.  This causes assertion failures in
DecoderBridge::Read().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/935

This eliminates a tiny amount of overhead because the compiler can
choose how to pass the parameter.

This fixes the Windows build.  Linking failed because some packages
(e.g. libFLAC) default to enabling `_FORTIFY_SOURCE`, which is broken
in recent mingw versions
(https://github.com/msys2/MINGW-packages/issues/5803).

While libsndfile doesn't like partial reads in the middle of a file
(see commit 95ac6071), it allows partial reads at the end of a file.
It doesn't pay attention to the file size when issuing a read.

Commit ecb67a1e (MPD 0.18.12) was a regression: previously,
partial reads at the end of a file were possible, but switching to
decoder_read_full() made this an error condition.  This way, a portion
at the end of each file was lost, leading to corruption with gapless
playback (https://github.com/MusicPlayerDaemon/MPD/issues/936).

This fix switches to the newly introduced function
decoder_read_much(), which does the same as the code before commit
ecb67a1e.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/936