Hide mouse pointer if surface is fully dimmed

[Elvyria]

Hello, my monitor is capable of automatically disabling backlight, if the surface is fully black, so i switched from using WM's dpms to dim and it works much more reliable, just my cursor is too flashy and monitor doesn't recognize surface as fully black, this PR should help with it without introducing new useless flags.

I can't imagine that someone would want to have mouse pointer on a fully black surface, but if you disagree, I can make it a flag for CLI.

[marcelohdez]

Nice! Could you add mentions about this behavior to the --alpha argument in the manpage and help string in src/opts.rs? e.g:

\-a, --alpha <ALPHA>
	Set the *alpha* value of the overlay, 0.0 being transparent and 1.0 being
+	solid black. When solid, cursor will be hidden. Default is 0.5.

    #[arg(
        short,
        long,
+       help = format!("0.0 is transparent, 1.0 is opaque. When opaque, cursor will be hidden. [default: {DEFAULT_ALPHA}]")
    )]

Also, it does not seem to work in Sway for some reason I can't figure out at the moment, but I will not let that hold the merge back, could figure that out later as it may need more invasive changes.

[Elvyria]

Looks good?

[marcelohdez]

LGTM, Thanks!