Move and refactor code from the Wavpack decoder plugin.

Replaces the method Close().

Fixes missing song length on high-latency Opus files.

According to tests with 320 kbit/s opus files with 60ms packets, we
need to skip up to 29 kB.

This corrects a major mistake from commit 724a59aa - there was one
small thing that commit was supposed to do, and it failed.
AV_TIME_BASE is not a seek flag.

Don't use the mapper - all DetachedSong instances we get have already
been mapped.

Merge duplicate code.

Fixes potential nullptr dereference.

Fixes potential nullptr dereference.

This pointer was allocated by libc, not by GLib.

Don't duplicate the buffer just to null-terminate the string.
According to libFLAC API documentation, the string is already
null-terminated.

To replace g_strdup().