sm501: Implement negated destination raster operation mode

Add support for the negated destination operation mode. This is used e.g.
by AmigaOS for the INVERSEVID drawing mode. With this change, the cursor
in the shell and non-immediate window adjustment are working now.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>