Fixes build failure after commit
0cea67ee

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.

There is no documentation on whether calling shout_metadata_add()
multiple times on one instance is allowed.  To be sure, let's allocate
the object on demand each time in SendTag().

This is more robust, for example it allows using the Shine encoder
plugin instead of LAME.

Coverity discovered that the Pulse plugin could throw exceptions from
Pause(), but that method was marked "noexcept" because its caller was
not designed to catch exceptions.  So instead of avoiding exceptions
(by catching and logging them in each and every implementation), let's
allow them, and do the catch/log game in the MPD core.

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Avoid accessing FilteredAudioFormat attributes directly.

Configure the AudioFormat after opening the encoder, because only now
we know the final format.

This order will be necessary for proper AudioFormat initialization,
because the encoder may change the format.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate dependency on io_thread_get().

The user manual specifies "encoder", which is consistent with other
output plugins.  "encoding" should be deprecated.

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.