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

Dan,

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

Your patch gives better results for the colours in colorcir.ps, but 
worse results for grey.  The new code is using a more restricted 3 
levels (white, mid-grey, black), unlike the old code which used 4 
levels for greys.  In addition, the near black rectangles look 
lighter than in 24-bit colour.    Although 4-bit colour is rarely 
used, I would like to see the behaviour for greys to be similar to 
the old behaviour.

I also tried to compare it with 8-bit colour, and found that this is 
seriously broken, and has been since at least GS 8.11.  If you use 
the display device with 8-bit colour, you get 4 levels for each 
colour, and 32 levels for greys.  So colorcir.ps in GS 7.07 gives 
solid colours for the grey circle, but halftones for the colour 
circles.  In GS 8.14 and CVS HEAD (even after your recent patch) the 
greys are halftoned and should not be.  Do you want a new bug opened 
for this?  Example using
  gswin32c -r72 -dDisplayFormat=16#30401 examples/colorcir.ps
Other useful formats are:
  16#30101 (monochrome)
  16#30401 (4-bit colour)
  16#30801 (8-bit colour)
  16#30802 (8-bit grey)
  16#30804 (24-bit colour)
If using Unix and gsx, change the 3 to a 1.