RE: [AD] improvements on alpha blending...

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

>>  - Replace all `/ 256' by `>> 8' (the compiler can't
>> do this automatically because x>>8 != x/256 if x<0).
>
>Blenders should use unsigned numbers. I believe most do.
>Shifts would get emitted by the C/C++ compiler then.

That's good, because I think replacing divisions by shifts would break
portability (Allegro is supposed to be portable to non-PC platforms, right?)