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

We already know the offset.

Don't abort the process if there's not enough memory.  This buffer is
not important and can be large.

Merge two free() calls.

A bug report was submitted with a 600 kB ID3 tag that could not be
read by MPD.

Here's a change to dynamically allocate the DSD ID3 tag buffer.
Pretty much anything with cover art is going to exceed the existing,
static 4k limit...  Here's a change to dynamically allocate the buffer
and sanity check it at some upper limit.  I rather arbitrarily pulled
256k out of thin air just to keep a corrupt file from causing it to
trying to allocate a buffer larger than available memory.

Variable-length arrays are not allowed in C++.

Reduce duplicate code.

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

Decouple some more from GLib.