The problem:
When rasterizing to an image, PDFs that use transparency aren't antialiased
correctly.

Description:
I've included 3 use cases that should look roughly the same.
All three contain:
A blue box
The word 'Hello' in green overlapping the blue box.
The word 'Hello' in blue, overlapping both the previous hello and the blue box.

To render, I use the following command:
gswin32c -dSAFER -dBATCH -dNOPAUSE -sDEVICE=png16m -dGraphicsAlphaBits=4
-dTextAlphaBits=4 -sOutputFile=foo.png foo.pdf

NoTransparency.pdf renders with flawless antialiasing.
TransparencyKnockout.pdf renders with no antialiasing.
TransparencyNoKnockout.pdf renders with no antialiasing.

(The same symptoms occur if I use ImageMagick to convert to GIF with the default
options.)

The antialiasing shows up correctly for all 3 cases in Adobe Acrobat Reader
versions 5 through 7.  For my application to meet spec, I need antialiasing in
my transparent PDF conversion.

Explanation of cases:
NoTransparency.pdf simply renders pink text on top of green text on top of the box.

TransparencyNoKnockout.pdf renders the blue box, a green rectangle masked by
text, and a pink rectangle masked by text.

TransparencyKnockout.pdf renders the blue box, and a knockout group containing
green text masked by text, and pink text masked by text.

Why would you mask text with text, you ask?  Because I might want that text to
be partially transparent, and I need the text to knock out to the "paper" (which
in this case is white with a blue box), not the text beneath.

A more complicated case with an image has been included as a kind of final check.

Note: (TransparencyNoKnockout and TransparencyKnockout may look slightly
dissimilar, because in latter, the pink text may antialias against the paper,
even where it overlaps the green text.)