Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Hide the definition from C code, to prepare the transition to C++.

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

Prepare for big CUESHEET tags.

The new method pair() receives an arbitrary name/value pair.  Support
for this is being added to a few decoder plugins.

Remove clutter from wavpack_scan_file(), and use a (large) fixed
buffer for the tag item.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

For MPD's new floating point support: when a decoded wavpack file
needs to be resampled, don't convert float to int and back to float.

Use local variable "is_float".

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

It used to ignore the decoder_data() return value.

libwavpack provides int32_t samples, and wants uin32_t for sample
counts.

.. and remove one indent level.

Don't allocate each replay_gain_info object on the heap.  Those
objects who held a pointer now store a full replay_gain_info object.
This reduces the number of allocations and heap fragmentation.

Pass the current URI to wavpack_open_wvc().

Use input_stream.uri.

Make it X_decoder_plugin.c.

This function replaces the replay_gain_info parameter for
decoder_data().  This allows the decoder to announce replay gain
changes, instead of having to pass the same object over and over.

Major API redesign: don't let the caller allocate the input_stream
object.  Let each input plugin allocate its own (derived/extended)
input_stream pointer.  The "data" attribute can now be removed, and
all input plugins simply cast the input_stream pointer to their own
structure (with an "struct input_stream base" as the first attribute).

Remove the data_time parameter from decoder_data().  This patch
eliminates the timestamp counting in most decoder plugins, because the
MPD core will do it automatically by default.