We are running into rasterization inaccuracies here.
This test draws green circles on a green rectangle and assumes that the result
will be solid green.
Here's what happens with D2D:
D2D computes coverage values in floating point so assume that we get a coverage
of about 0.112399 or 0.112399*255 == 28.6
green (128/255 == 0.50196) is multiplied by the coverage to get 0.0564702 or
.0564702*255 == 14.39
the results of the pixel shader get rounded before blending. So we get
(r=0,g=14,b=0,a=29)
blending this OVER green gives:
14+(255-29)*128/255 == 127 instead of 128.
This doesn't happen with other backends because they typically use integer
coverage amounts.
We fix it by using ' ' instead of '.'
