Lunatic: rewrite shadexfog.create_128_trans to allow creating fewer tables.
It must be a power of two in [2 .. 128]. Rewrite test/sprite_access.con's
liztroop example to account for that.
Also, add function shadexfog.create_trans() which aceepts a function
(r,g,b,R,G,B) -> blended (r,g,b) and rewrite shadexfog.create_additive_trans()
in terms of it.