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.

bad_alloc depends on <new>