SEEK_SET is defined by unistd.h.  Explicitly include it.

Refuse to play audio formats which are not supported by MPD.

The ov_info() return value may be corrupt when ov_read() did not
return a positive value.  First check for success, then check
ov_info().

Unfortunately, ov_fopen() is not supported by libvorbis versions older
than 1.2.

Due to an unnoticed merge error, there was a superfluous "}".  Remove
it.

Move the reponsibility for freeing the file handle to libvorbis.

Eliminate two decoder_get_command() invocations from the main loop.

Don't let the buffer grow until it is full, flush it whenever there is
data available.

The stream_decode() and file_decode() methods returned a boolean,
indicating whether they were able to decode the song.  This is
redundant, since we already know that: if decoder_initialized() has
been called (and dc.state==DECODE), the plugin succeeded.  Change both
methods to return void.

Having an array instead of individual variables allows the use of the
replay_gain_mode enum as an array index.

Some code simplification.  Avoid pointers to pointers.

Renamed functions and variables.

Instead of having a seprate try_decode() method, let the
stream_decode() and file_decode() methods decide whether they are able
to decode the song.

Instead of checking the stream_types bit set, we can simply check
whether the methods stream_decode() and file_decode() are implemented.

If an input stream provides tags (e.g. from an icecast server), send
them in the decoder_data() and decoder_tag() methods.  Removed the
according code from the mp3 and oggvorbis plugins - decoders shouldn't
have to care about stream tags.

This patch also adds the missing decoder_tag() invocation to the mp3
plugin.

Ogg and ffmpeg detection was disabled when the stream was not
seekable, because the detection was too expensive.  Since the curl
input stream can now rewind the stream cheaply, we can re-enable
detection on streams.

Provide an API for submitting additional tags from the stream.

Don't pass the "seekable" flag with every decoder_data() invocation.
Since that flag won't change within the file, it is enough to pass it
to decoder_initialized() once per file.

The strings were constant, but the pointers weren't.  C syntax is
somewhat tricky..

All decoder_plugin structs are initialized at compile time, and must
never change.

Don't return 0/-1 on success/error, but true/false.  Instead of int,
use bool for storing flags.

A decoder_flush() invocation was missing in the FLAC plugin, resulting
in casual assertion failures due to a wrong assumption about the last
chunk's audio format.  It's much easier to remove that decoder_flush()
function and make the decoder thread call ob_flush().

Call ob_clear() in decoder_command_finished() instead of implementing
that call in every decoder plugin.

For boolean values and success flags, use bool instead of integer (1/0
for true/false, 0/-1 for success/failure).

Renamed all functions and variables.

Everybody should use struct input_stream.

These plugins are not input plugins, they are decoder plugins.  No
CamelCase in the directory name.

Don't compile the sources of disabled decoder plugins at all, and
don't attempt to register these.

The last bit of CamelCase in audio_format.h.  Additionally, rename a
bunch of local variables.

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

The old struct initializers are error prone and don't allow moving
elements around.  Since we are going to overhaul some of the APIs
soon, it's easier to have all implementations use C99 initializers.

Get rid of CamelCase, and don't use a typedef, so we can
forward-declare it, and unclutter the include dependencies.

Getting rid of CamelCase; not having typedefs also allows us to
forward-declare the structures.

The code said "decoder_command==STOP" because that was a conversion
from the old "dc->stop" test.  As we can now check for all commands in
one test, we can simply rewrite that to decoder_command!=NONE.

Return early when the player thread sent us a command.  This saves one
level of indentation.

On our way to stabilize the decoder API, we will one day remove the
input stream functions.  The most basic function, read() will be
provided by decoder_api.h with this patch.  It already contains a loop
(still with manual polling), error/eof handling and decoder command
checks.  This kind of code used to be duplicated in all decoder
plugins.