Rename luma table to delta table and change how it is used.

CC: libav-stable@libav.org
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Diego Biurrun <diego@biurrun.de>
(cherry picked from commit f8c34f4b)
(cherry picked from commit 73f3c8f7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>