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

Allows more compiler optimizations.

Replacing GLib macros.

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

Replaces GLib's GError.

Avoid duplicating the tag.

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

Move code from the Vorbis decoder plugin.

Internally the vorbis (non-Tremor) decoder is working in floating
point, and it's not really necessary to cut the output back to 16-bit
if the soundcard or OS supports higher resolution.

The decoder can be trivially modified to bypass its internal
quantisation and produce floating-point output, and a separate
quantisation can be used as appropriate to the platform.

Call decoder_get_command() before doing anything else.