us to get rid of the entire b-frame buffer and reduce some of the
cache footprint. You should see a 10 to 15% improvement in speed.
I also added hooks to allow the display layer to allocate the
working image buffers. This will allow us to do fancy things with
AGP transfers (and speed up the Xv stuff) later.

NB. the 3dfx, SDL, and gatos drivers are broken as they need
draw_slice, flip_page, and allocate buffer implemented. Also mga_vid will
flicker a bit we don't do proper hardware page flipping yet.

Have fun.

cheers,
aaron