Fix for 687390 GhostScript 8.11 inserts rendering artifacts with GIFs as source

Fix for 687390 GhostScript 8.11 inserts rendering artifacts
with GIFs as source.  The actual problem is an error which
resulted in a pale yellow being shown as a pale pink.  This
occurs when dithering on devices which use a 256 color palette.

DETAILS

The halftoning logic assumes that there are the same number
of shades of each color.  However the 256 color palette
devices were using a 7x7x5 color cube.  I.e. there were 7
shades of red and green, however there were only 5 shades of
blue.  The data in the device's color_info (max_color, max_gray,
dither_colors, and dither_grays) indicated that there were
7 shades.  The halftoning logic, using this information, was
attempting to display the given pale yellow color by dithering
between 6/7ths and 7/7ths of each component.  However the
encode_color routine returned colors which include solid blue
for all cases.  This biased the resulting halftoned color
toward blue.

The fix involves changing the 256 color palette devices to
use a 6x6x6 color cube.  The same routine pc_8bit_map_rgb_color
is used for all of the 256 color palette devices.  Thus the
change consists of modifying this routine and the associated
pc_8bit_map_color_rgb routine.  The specification of each of
the devices has to be changed to represent the change in the
cube size.

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