Negating an unsigned integer does not work.

For type safety and code readability.

Avoid roundtrips to floating point.

This check was redundant, because we could only exit the loop when
ret==DECODE_OK.

Not necessary.

Reduce header dependencies.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Move code from the "mad" plugin.

Move code from the "mad" plugin.

Replaces GLib's g_str_has_prefix().

Replaces GLib's g_ascii_strcasecmp().

Decouple some more from GLib.

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