Use luminance formula for random colors in example

[CHiPs44]

cf. https://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale:

luminance = 0.2126 * r + 0.7152 * g + 0.0722 * b

Here we have r, g and b from 0 to 31 and no FPU...

[CHiPs44]

NB: 0.2126 is 2126 / 10000, and so on.

luminance = (2126 * r + 7152 * g + 722 * b) / 100

should be between 0 and 255 if 8 bits and comparable across 2 colors.

[CHiPs44]

Formula used for console16 palette, see #26.

[CHiPs44]

Have another byte array alongside each palette with luminance values?

Have another byte array alongside each palette with indexes of colors ordered by luminance?

Compute luminance at runtime and keep it another array? Sort it?

Handle only 16 color palettes? 16 and 256 ones? (2 and 4 colors palettes are less used)

NB: with 256 colors, collision may happen more frequently, perhaps compute luminance with 16bits instead of 8.