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

Avoid while(true) loops, and convert them to a loop with a proper
condition.

Since the aac and mod plugins have told MPD that they cannot seek, MPD
will never send a SEEK command to them.  Removed the SEEK comand
checks from both plugins.

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().

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.

When there are standardized headers, use these instead of the bloated
os_compat.h.

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.

Seeing the "mpd_" prefix _everywhere_ is mind-numbing as the
mind needs to retrain itself to skip over the first 4 tokens of
a type to get to its meaning.  So avoid having extra characters
on my terminal to make it easier to follow code at 2:30 am in
the morning.

Please report any new issues you may come across on Free
toolchains.  I realize how difficult it can be to build/maintain
cross-compiling toolchains and I have no intention of forcing
people to upgrade their toolchains to build mpd.

Tested with gcc 2.95.4 and and gcc 4.3.1 on x86-32.

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.

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.

We need the decoder object at several places in the AAC plugin.  Add
it to mp3DecodeData, so we don't have to pass it around in every
function.

Copy some code from aac_decode() to aac_stream_decode() and apply
necessary changes to allow streaming audio data.  Both functions might
be merged later.

initAacBuffer() should really only initialize the buffer; currently,
it also reads data from the input stream and parses the header.  All
of the AAC buffer code should probably be moved to a separate library
anyway.

The AAC plugin sometimes does not check the length of available data
when checking for magic prefixes.  Add length checks.

Eliminate some duplicated code by using fillAacBuffer().

Find AAC frames in the input and skip invalid data.  This prepares AAC
streaming.

adts_check_frame() checks whether the buffer head is an AAC frame, and
returns the frame length.

Shifting from the buffer queue is a common operation, and should be
provided as a separate function.  Move code to aac_buffer_shift() and
add a bunch of assertions.

When checking for EOF, we should not check whether the read request
has been fully satisified.  The InputStream API does not guarantee
that readFromInputStream() always fills the whole buffer, if EOF is
not reached.  Since there is the function inputStreamAtEOF() dedicated
for this purpose, we should use it for EOF checking after
readFromInputStream()==0.

Fill the AacBuffer even when nothing has been consumed yet.  The
function should not check for consumed data, but for free space at the
end of the buffer.

Return instead of putting all the code into a if-closure.  That saves
one level of indentation.

adtsParse() always returns 1, and its caller does not use the return
value.

Since we eliminated the parameters retFileread and retTagsize in all
callers, we can now safely remove it from the function prototype.

"decoder plugin" is a better name than "input plugin", since the
plugin does not actually do the input - InputStream does.  Also don't
use typedef, so we can forward-declare it if required.

Since we want to hide mpd internals from the decoder plugins, the
plugins should not check dc->state whether they have already called
decoder_initialized().  Use a local variable to track that.

Provide access to seeking for the decoder plugins; they have to know
where to seek, and they need a way to tell us that seeking has failed.