fix: incorrect line opacity mapping

[N-R-K]

current code treats opacity as [0, 100] even though the manpage documents it as being [0, 255].

Fixes: #281

[N-R-K]

It should fix the issue, but some actual real-world testing is still needed.

[guijan]

The opacity setting doesn't seem to work in general:

scrot -s -l width=4,opacity=1

I see no difference between master and this PR, both are broken, the line isn't transparent at all, it does blend oddly with whatever is under it however.

Why not just remove it since nobody complained about it not working in all this time?

[N-R-K]

The opacity setting doesn't seem to work in general:

Are you running under a compositor that supports _NET_WM_WINDOW_OPACITY?

scrot/man/scrot.txt

Lines 163 to 164 in 6e2b522

	100% transparent. For the opacity of the line this is only
	effective if a Composite Manager is running.

since nobody complained about it not working in all this time?

It seems to be working fine (except the 100 vs 255 part) for the person that opened the bug report #281:

Instead, opacity=255 is slightly see through, but opacity=100 is completely opaque.

[guijan]

I'm running xcompmgr, seems it doesn't. picom (a fork of xcompmgr) also doesn't it seems.

[guijan]

Should probably document that it can't be any compositor, too.

[N-R-K]

picom (a fork of xcompmgr) also doesn't it seems

What WM were you running? Perhaps this flag makes it work?

https://github.com/yshui/picom/blob/3aed5599c3f73cbfa53b0249795e76ab07cf9ecd/man/picom.1.asciidoc?plain=1#L130C1-L131

[guijan]

spectrwm. Still doesn't work with that flag.

[N-R-K]

If no one can test this, I'll just merge it in a day or two, since the math adds up.