MAddMode ; Returns value 7 which represents `add' mode in the blit modes

Returns value 7 which is the new `add' mode. This affects all drawing commands,
blits, scrolls and particles which allow you to select a rendering mode. In Add
Mode, the pixels of the source and destination images, or alternatively the
pixels of the destination and ink colour, are simply added together and the
result placed in the destination. To cause a `subtraction' effect you can simply
add 256-Value in order to achieve the same effect. Add mode can be used for
simple software colour cycling and for faster shade-bob type effects, for
example.