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 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=620119
Refactor the DVD subpicture compositing, switching it to 8-bit alpha
calculations. Reuse some of the resulting code to implement PGS
subpicture blending.
Implement parsing and collecting of composition objects properly, but
assuming a single active window and colour palette for now. I need more
PGS samples.
