Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

Variable-length arrays are a C-only feature.

Since we now don't duplicate all items, we can easily remove the 64kB
limit from OpusReader::ReadString() and instead silently ignore and
skip all strings which are longer than 4 kB.

This fixes a tag duplication bug with Opus file containing a very long
`METADATA_BLOCK_PICTURE` tag, which occurred because the Opus plugin
returned false after parsing all tags, and then the MPD core fell back
to FFmpeg which scanned the tags again.

Prepare for templated functions.

This plugin is interesting only for a tiny fraction of MPD users, so
let's not spam everybody else's log with it.

The `tag_builder.empty()` check was wrong for the SubmitReplayGain()
call.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/497

Don't catch and log exceptions.  Let the caller handle the error.

This mime type is to enable the AlsaInputPlugin to communicate the pcm stream audio format to the decoder

Re-add build support for the fixed-point Vorbis (Tremor) decoder, which
was dropped when switching from Autotools to Meson.

Note, that it is not possible to build both, the Vorbis and the Tremor
decoder.

Closes: #405

Closes https://github.com/MusicPlayerDaemon/MPD/issues/456

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

`AVCodecParameters` contains values from the codec detected by
avformat_find_stream_info(), but after avcodec_open2(), a different
codec might be selected with a different `AVSampleFormat`.  This leads
to misinterpretation of data returned from FFmpeg, leading to random
noise or silence.

This was observed with FFmpeg 4.0.2 and a TS container file containing
MP2.  A mp3-float codec was detected returning `AV_SAMPLE_FMT_FLTP`,
but finally the `mpegaudiodec_fixed.c` was used, returning
`AV_SAMPLE_FMT_S16`.

By using the audio format from `AVCodecContext`, we ensure that MPD
and FFmpeg always agree on the actual audio format in the buffer.

This removes the FFmpeg bug workaround from commit e1b032cb which I
assume is obsolete after 7 years.

Fixes #380

Drop some compatibility code.

Closes #360