New base class for other InputStream implementations that run in the
I/O thread.

Fixes assertion failure.

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

Allows more compiler optimizations.

Move and refactor code from the Wavpack decoder plugin.

Eliminate duplicate code.

Allow compiling test programs with only selected plugins.

Caused by commit 0b4fa41a

Make attributes const, don't allow partial initialization.
Instantiate the CdioParanoiaInputStream object at the end of
input_cdio_open().