Fix for 687403 Poor halftoning for 16 color (4 bit) devices

Fix for 687403 Poor halftoning for 16 color (4 bit) devices.

DETAILS:

This bug is a brother to bug 687390.  That bug was for 256
color (8 bit) devices.  Russell Lang suggested test the 4 bit
devices and they were found to have problems caused by the same
logical error.  The halftoning logic assumes that all components
for a device have the same number of shades.  This was not true
for the implementations of the 4 and 8 bit color devices.
This fix converts the 16 color devices to using a 2x2x2 cube.

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