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.

Eliminates some duplicate code, and as a side effect, this works
around clang 3.8 compiler warning because a negative value was
shifted.

This was lost in commit 8ead8f7e, because this was never implemented
for the new API.

Allow the compiler to invalidate buffer contents.

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

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

Replaces GLib's GError.

This old version has been obsolete for many years - time to remove it.