Implements Mantis ticket 0003340.

Internally the vorbis (non-Tremor) decoder is working in floating
point, and it's not really necessary to cut the output back to 16-bit
if the soundcard or OS supports higher resolution.

The decoder can be trivially modified to bypass its internal
quantisation and produce floating-point output, and a separate
quantisation can be used as appropriate to the platform.

Call decoder_get_command() before doing anything else.

This lives in protocol/result.h now.

Eliminate the hack in handle_idle().

Just use the values assigned by the compiler.

We use range-based for and nullptr, which was not implemented prior to
gcc 4.6.

Conflicts:
	src/locate.c

These are confusing, and since MixRamp development has ceased, not
useful to anybody.

libvorbisidec and libvorbis export the same symbols, which is a
dangerous thing.  Since libvorbisenc depends on libvorbis, this can
get nasty, so let's disable the Vorbis encoder unless the user
explicitly wants it.

WavPack tags are always APEv2, by definition.  Reuse the tag_table
from tag_ape.c, instead of rolling our own.

Avoids clang pickiness.  This code is not correct, but we'll fix that
another day.

Using libopus and libogg.

Merge duplicate data.

When visiting a song, GetSong() was called, but this object was never
returned by calling ReturnSong().  This patch locks the database only
once in Visit() and passes the original song object to the visitor,
avoiding the copy.

Allow the function to be cancelled.

Fixes potential access to uninitialised memory.

This library crashes on out-of-memory (NULL pointer dereference).
There's not much useful MPD can do in such a situation, so let's
explicitly abort instead, just like GLib does.