The isCompatibleRaster() method in DirectColorModel requires that the
DirectColorModel have the same number of pixelbits as the Raster's
SampleModel. Unfortunately, since the SampleModel will calculate the
minimum number of bits required to hold all of the sample masks and since
the DirectColorModel allows the pixelbits given to it to exceed that number,
the two might not match for a DirectColorModel with an overly generous
value for pixelbits.