RasterizeTrapezoid screen function. These hooks will be called for
    imprecise, non-sharp trapezoids with A8 destinations.
Note that the current main consumer of trapezoids, cairo, is requesting
    precise, sharp trapezoids by not changing the default Picture
    attributes, but gets non-sharp effects in software because fb bases its
    choice of sharp/non-sharp on the mask format being A8 vs A1, and cairo
    asks for A8. Follow fb's (poor?) example by ignoring the sharp setting
    and basing the choice off of the mask being A8.