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.
Add setcaps logic on the subpicture sink pad for configuring
which subpicture format is arriving.
Add the first piece of PGS subpicture handling by dumping the stream
contents out to the terminal as the packets arrive.
Add some more debug.
Don't calculate the running time for our subpicture packets twice,
once is enough.
