Compute proper foreground color for labels

[clarfonthey]

This properly ensures that the contrast ratio is maximised for any given color choice.

The comments explain the exact calculations done (you can check that I transcribed it correctly), but to fully clarify for review:

1. Human eyes are better at detecting contrast than absolute differences in brightness. So, it's easier to see the difference between 0% and 10% brightness, than 90% and 100% brightness, even though it's the same absolute difference.
2. As a result, sRGB scales the colours so that the 0-255 range for each component has more dark shades than bright shades.
3. If you want to check the contrast between two colours, you need to undo this scaling.

[silverwind]

Note there is a similar function in JS that should be made to match the backend implementation:

gitea/web_src/js/components/ContextPopup.vue

Lines 75 to 86 in c9bca8c

	labels() {
	return this.issue.labels.map((label) => {
	const red = parseInt(label.color.substring(0, 2), 16);
	const green = parseInt(label.color.substring(2, 4), 16);
	const blue = parseInt(label.color.substring(4, 6), 16);
	let color = '#ffffff';
	if ((red * 0.299 + green * 0.587 + blue * 0.114) > 125) {
	color = '#000000';
	}
	return {name: label.name, color: `#${label.color}`, textColor: color};
	});
	}

[clarfonthey]

Good catch! I'll fix that this evening

[clarfonthey]

Fixed!

[jolheiser]

I think this all looks great, would you be able to attach a couple example images?

[clarfonthey]

Sure thing! I'll take some tonight with some sample colours.

[jolheiser]

Disregard my other comment, for some reason I thought this was changing colors, rather it's just a different formula for choosing white/black text.

[clarfonthey]

I'll still take some tonight anyway, since it might be helpful for the changelog