Remove GLib dependency from some headers.

Enable gcc optimizations.

<stdbool.h> needs to be included unconditionally from definition of
NDEBUG, since »bool« doesn't get defined otherwise.
Signed-off-by: Andreas Wiese <aw-devel@meterriblecrew.net>

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

In !NDEBUG, remember which audio_format is stored in every chunk and
every pipe.  Check the audio_format of every new data block appended
to the music_chunk, and the format of every new chunk appended to the
music_pipe.

music_pipe_peek() is similar to music_pipe_shift(), but doesn't remove
the chunk.  This allows it to be used with a "const" music_pipe.

Turn the music_pipe into a simple music_chunk queue.  The music_chunk
allocation code is moved to music_buffer, and is now managed with a
linked list instead of a ring buffer.  Two separate music_pipe objects
are used by the decoder for the "current" and the "next" song, which
greatly simplifies the cross-fading code.

Added music_pipe_allocate(), music_pipe_push() and
music_pipe_cancel().  Those functions allow the caller (decoder thread
in this case) to do its own chunk management.  The functions
music_pipe_flush() and music_pipe_tag() can now be removed.

The function is unused, since decoder_api() uses music_pipe_write() /
music_pipe_expand() now.

This new API gives the caller a writable buffer to the music pipe
chunk.  This may allow the caller to eliminate several buffer copies,
because it may manipulate the returned buffer, until it calls
music_pipe_expand().

When a CANCEL command is received, the player should drop all chunks
of the next song.  Added new funciton music_pipe_chop() which is used
for that.

Provide a debug function which asserts on the sample format of all
chunks.  Call this function in each iteration of the player main loop.

Each music chunk can now carry a tag object.  Decoder plugins which
support it (e.g. oggvorbis) may use this to inject decoded tags into
their output.

Add doxygen compatible comments.

The "volatile" keyword doesn't help here, because we have proper
memory barriers, but it disables some optimizations.  Remove it.

The name "num_chunks" expresses the meaning of the variable better.

Rename all variables and struct members.

Last music_pipe rename patch: renamed the global variable (singleton).

Replace all direct music_pipe struct accesses with wrapper functions.
The compiled machine code is the same, but this way, we can change
struct internals more easily.

.. and rename dc.audioFormat to dc.in_audio_format.  The music pipe
does not need to know the audio format, and its former "audioFormat"
property indicated the format of the most recently added chunk, which
might be confusing when you are reading the oldest chunks.

Rename all functions to the new prefix.

.. and rename ob_chunk to struct music_chunk.

No CamelCase in the file name.  The output_buffer struct is going to
be renamed to music_pipe.  There are so many buffer levels in MPD, and
calling this one "output buffer" is wrong, because it's not the last
buffer before the music reaches the output devices.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

The function ob_is_empty() is called very often.  It's worth it to
convert it to an inline function.

"bool" should be used in C99 programs for boolean values.