Enclosed is a simple test case for the "black spots"
sampling errors that have been reported off-and-on.
The world.rib file contains a single NuPatch arranged
to create a disk, while sample_buster.sl is an imager
shader that does a straight color assignment to any
pixel with an alpha < 1. By rendering geometry with
the constant shader and a contrasting color, any
sampling errors or cracks in the geometry show-up as
isolated pixels - normally, these types of errors would
tend to be masked by filtering and oversampling.