The voice register functions return the same voice structure
upon multiple registration. It causes us two problems:

If we delete a voice without deregistering it, it leaves
a dangling pointer inside the library.

If we delete or unregister a voice at uninit, it may still
be in use by another instance of the filter.

The second problem is solved by keeping an usage counter inside
asrc_flite. This is not thread-safe, but neither is flite itself.