swrast: add TFP support to swrast.

swrast: add TFP support to swrast.

This adds TFP support to the swrast driver, with this I can run gnome-shell inside Xephyr slowly. I've no idea why I did it, and g-s has other rendering issues under swrast, but it might be useful to hook up llvmpipe later. I've no idea if I even want to commit it at this point.

An enhanced version might just pass the pointer in the indirect rendering case
and avoid the memcpy.

Signed-off-by: Dave Airlie <airlied@redhat.com>