Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

For type safety and code readability.

Move to fixed-point integers instead of floating point.

Move code from the "mad" plugin.

Move code from the "mad" plugin.

Move and refactor code from the Wavpack decoder plugin.

Eliminate duplicate code.

Replaces the method Close().

Avoid GLib.

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Move code from the "mad" plugin.

Move code from the "mad" plugin.