Send a 256 row * 3 column color lookup table to the Datapixx.
The three columns contain the red, green, and blue components respectively. The
data are in the range 0-1, and map to the Datapixx 16-bit video DAC values, with
0 mapping to a DAC value of 0, and 1 mapping to a DAC value of 65535. The color
lookup table is used by the Datapixx when in video mode LUT_48, and for the
overlay when in video mode MONO_16. The CLUT is sent to the Datapixx over USB,
and is copied to the hardware CLUTs during the next vertical blanking interval.