Dithering selects between two color values or indices.
In RGBA mode, consider the value of any of the color
components as a fixed-point value with **m** bits to the
left of the binary point, where **m** is the number of bits
allocated to that component in the framebuffer;
call each such value **c**.
For each **c**,
dithering selects a value such that
(after this selection, treat as a fixed point value in [0,1] with
**m** bits).
This selection may depend on the and coordinates of the pixel.
In color index mode,
the same rule applies with **c** being a single color index.
**c** must not be larger than the maximum value representable
in the framebuffer for either the component or the index, as appropriate.

Many dithering algorithms are possible,
but a dithered value
produced by any algorithm must depend
only the incoming value
and the fragment's **x** and **y** window coordinates.
If dithering is disabled, then each color component is truncated
to a fixed-point value with as many bits as there are in the
corresponding component in the framebuffer; a color index
is rounded to the nearest integer representable
in the color index portion of the framebuffer.

Dithering is enabled with ** Enable** and disabled with ** Disable**
using the symbolic constant ` DITHER`.
The state required is thus a single bit.
Initially, dithering is enabled.

Sat Mar 29 02:23:21 PST 1997