Add a set of three hooks for accelerating trapezoids, and use it for the
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.