Combining these two in one single value saves some memory, but is
complicated and we may need the src_sample_format for new features
later.

This reverts commit 2c3eeb71.  This
workaround has been obsoleted by commits
bf26adf5 and
32380d1d

Similar to commit 32380d1d, these are
the final parts for really fixing
https://github.com/MusicPlayerDaemon/MPD/issues/469

Closes https://github.com/MusicPlayerDaemon/MPD/issues/577

These options are exclusive.

First part of the "real" fix for
https://github.com/MusicPlayerDaemon/MPD/issues/469

Preparing to add more state.

The dop_buffer will be moved out soon.

Reduce header dependencies.

Returning an empty buffer is a normal result now (since commit
79839db3), and doesn't deserve such a
big comment.

Workaround for a regression caused by commit
a06bf388, revealing a problem with
discarding odd numer of frames in the DSD_U32 and DoP converters,
causing distortions with DSD_U32 and DoP on 32 bit CPUs.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/469

This can happen if the DoP converter doesn't get enough source samples
for one destination quad.  This isn't a critical bug, because the OSS
plugin doesn't support DoP yet, but it's good to be prepared.

Instead of passing tag and group, pass an array of tags.  To support a
nested return value, return a nested std::map of std::maps.  Each key
specifies the tag value, and each value may be another nesting level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/408

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.

This can be used for other purposes as well.