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.

Replaces GLib's GError.

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