The code to choose a pipeline only chooses the optimized rendering
code if the alpha mode is Src or SrcOver, with no extra alpha, and
the color is opaque.

If the alpha mode is Src (no extra alpha) and the color is non-opaque,
the same optimized rendering code can be used since this only affects
the pre-calculated pixel value stored for each rendered pixel, but
alpha blending code is chosen instead, reducing performance.

Even worse, for indexed images the alpha blending code does an extremely
poor job of choosing the best pixel for the results of blending (it only
examines the opaque pixels in a lookup table for performance reasons)
and so the blending code will produce much worse results than using the
optimized rendering code with a carefully chosen pixel in these cases.

###@###.### 2005-03-31 21:42:00 GMT