[request] Add the Superimposer

[sdsnatcher]

Except for the Video9000, none of the later V9990 cartridges had the superimposer to allow the V9990 image to be placed over the V99x8 image.

Please add this feature, because:

1. It allows for much richer games for low cost, using the V9958 as a colorful 3rd background plane
2. It makes it easier to use, since it won't be necessary to switch between two unrelated video outputs anymore

[cristianoag]

Totally agree with this. However depending on the size of the circuitry we need to do superimpose we may need to extend the size of the cartridge (taller). That will require a custom cartridge box.