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 M16. The CLUT is sent to the Datapixx over USB, and
is copied to the hardware CLUTs during the next vertical blanking interval.