filters: ditherings: Enable all pixel types.

Make ditherings work with all pixel types.

Add conversion from any pixel type to RGB888 before the dithering is
done. While this is not 100% correct solution it's good enough for the
time being.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
spiv: Add forgotten -e config opt

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
backends: Implement ResizeAck() for virtual backend.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>