feat: enforce minimum contrast ratio for reverse video cursor

[jameshurst]

This PR ensures the reverse video cursor meets a minimum contrast ratio, otherwise the default fg/bg is used instead.

The minimum contrast ratio of 2.5 was chosen from the kitty implementation linked in #2861.

The SrgbaTuple::contrast_ratio implementation also appeared to be incorrect since it was comparing lightness instead of luminance.

This should resolve #2861.

Before

After

[wez]

Thanks for this!

Let's add a config option to control that contrast ratio. There will also need to be an entry in the docs for that page, you can add one to https://github.com/wezterm/wezterm/tree/main/docs/config/lua/config

Thanks again!

[wez]

I think we should make this an option because I just know that someone is going to complain about this otherwise, regardless of how well selected the default value might be!

I'd suggest adding an option named something like cursor_minimum_contrast: f32 over here:

wezterm/config/src/config.rs

Lines 612 to 613 in b251509

	#[dynamic(default = "linear_ease")]
	pub cursor_blink_ease_out: EasingFunction,

You'll also need to add a little helper function to return the 2.5 default value and assign that using #[dynamic(default = "default_contrast_ratio")] as an attribute on that option.

then I think you should be good to examine self.config.cursor_minimum_contrast in this function instead of the constant.