Our SMask contains a fill of white square, and a constant alpha of 0.33

The PDF code looks as follows:

  /alpha  gs
  /redsquare Do
  /greensquare Do


Old code:

  1 btm
  3   fill white square
  5 etm
  6 btg
  8   fill red square
  A etg
  B
  C btm
  E   fill white square
  G etm
  H btg
  J   fill green square
  L etg

New code: Essentially the same sequence of events, but with extra knockout
groups around the fills:

  1 btm                          \
  2   btg                        |
  3     fill white square         > Call this mask M1
  4   etg                        |
  5 etm                          /
  6 btg
  7   btg
  8     fill red square
  9   etg
  A etg
  B
  C btm
  D   btg
  E     fill white square
  F   etg       <-----------------  Problem occurs here.
  G etm
  H btg
  I   btg
  J     fill green square
  K   etg
  L etg

When the code comes to do the etg highlighted above, it finds that M1
is still in force.