Shadow: Switch the Amiga/Atari bitplane code to the canonical X.Org license

Also add my name to the list of authors in COPYING

Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Keith Packard <keithp@keithp.com>