Closes https://github.com/MusicPlayerDaemon/MPD/issues/1294

The ChainFilter class is extremely complicated code, and will grow to
be even more complicated when the Filter interface gets extended.
Let's just remove it; we can easily chain many TwoFilters instead.

Insert an "aformat" filter which converts double-precision to
single-precision.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1235

If DetectFilterOutputFormat() fails to determine the output format,
insert an "aformat" filter which attempts to force a specific output
format.

Fixes part 2 of of https://github.com/MusicPlayerDaemon/MPD/issues/1235

Merge some duplicate code.

Since AVFilterContext are freed automatically, this wrapper class
serves no purpose.  Let's remove it.

The latter is easier to read and is the "correct" thing to do.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with cppcoreguidelines-special-member-functions
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Some FFmpeg filters change the sample format, and since MPD assumes
this never happens, this results in loud noise instead of music.  This
commit finally implements the TODO comment by sending one frame of
silence to the filter and checking the output frame's format.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1009

If no conversion is necessary, return the child Filter as-is.  This
allows removing all nullptr checks from AutoConvertFilter.

If the FFmpeg filter outputs planar data, interleave it, just like the
FFmpeg decoder plugin does.

filter_chain_parse() passes a temporary string pointer which results
in a use-after-free in the PreparedChainFilter::Child::Open() error
message.

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former is deprecated by C++14. The standard says they are the same:

The header defines all types and macros the same as the C standard library
header<stdint.h>.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-use-override
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Applying software volume to S16 samples means several bits of
precision are lost; at 25% volume, two bits are lost.  Additionally,
dithering adds some noise.

The problem gets worse when you apply the software volume code twice:
for the software mixer volume, and again for the replay gain.  This
loses some more precision and adds even more dithering noise, which
can become audible (see
https://github.com/MusicPlayerDaemon/MPD/issues/542).

By converting everything to 24 bit, we need to shift only two bits to
the right instead of ten, losing nearly no precision, and dithering is
not needed.  Even if the output device is unable to play S24 directly,
we can convert back to S16 with only one stage of dithering.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/542

Prepare for a new mode which may convert to a different sample format
when applying volume, to reduce dithering.

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.