http://dvd.sourceforge.net/spu_notes does not mention that high bits
are to be masked, and not clearing them makes a sample work, where
clearing them yielded left > right.
History from the dvdspu plugin, from where this code was copied,
does not shed any light, as tracing this code's origin shows
the same bitmasks being there in 2007 when it was imported.
https://bugzilla.gnome.org/show_bug.cgi?id=657091
