Compatibility: VGA
Expects: AX    1015H
BX    color register to read (0-255)
Returns: CH    green value (00H-3fH)
CL    blue value  (00H-3fH)
DH    red value   (00H-3fH)
Info: This obtains the current 18-bit RGB value of a specified DAC
color register.

See INT 10H 1010H for background info on 18-bit RGB values for
DAC color registers.

Notes: Use INT 10H 1017H to obtain the RGB values of a block of DAC

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INT 10H 1015H: Read One DAC Color Register