Palette DAC conversion fixes #1714
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The original VGA DAC values are 1, 5, 16 (decimal). 64.5/255 < 16/63 < 65.5/255 Therefore 65 (0x41) is a better approximation for the blue component. The rest of the text mode palette had similar inaccuracies, which were fixed in PR chocolate-doom#1659 (issue chocolate-doom#1658).
The difference is in the two least-significant bits.
Also includes the corresponding gamma decoding values for each gamma correction level.
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First two commits deal with conversion from 6-bit VGA DAC color values to 8-bit RGB. The original routine shifted the bits to the left, leaving the two bottom bits cleared, which results in a slight bias towards black in the output. This PR remedies this by using a linear formula that maps 0-63 to 0-255. Admittedly, this is a very pedantic fix with almost no discernible visual difference.
Third commit documents how
gammatablewas generated.