Clean up debug code for drawing highlight rects which wasn't even
compiling previously.
TODO: might be nice if this could be controlled by properties or
env variables to make it possible to enable the debug code at
runtime.
https://bugzilla.gnome.org/show_bug.cgi?id=667223
Conflicts:
gst/dvdspu/gstspu-vobsub-render.c
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.