implementing unfocused-window-opacity and unfocused-window-fill

[lgouger]

There's a feature in iTerm2 that I miss, so I'm exploring what it would take to add it to Ghostty.
The feature is iTerm's "Settings", under "Appearance" and "Dimming", it's a checkbox titled "Dim background windows".

I set out to implement this feature in Ghostty. Started by using Claude Code in Plan mode.
I've shared the plan file in a notes directory on my branch of the ghostty repo. (listed below)
After developing the plan, I asked Claude Code to implement it. It got something working, but it failed to capture the behavior I wanted when the new config params were not present.
I wanted the app to behave just as it does today, without this feature, if the config parameters aren't present.
After a bit of course correcting with Claude, I got a version that was ok, but used BLACK as the fill color
when applying the the overlay rectangle of the specified opacity. I then went about manually adding another
parameter unfocused-window-fill that defaults to the background-color. I modeled this on the unfocused-split-fill implementation.

I don't think this change is ready for a PR, since it's only implemented for macos, but I would like feedback.

My branch is named lpg/feature/unfocused-window-opacity on my fork of the ghostty repo,
https://github.com/lgouger/ghostty

The plan developed by Claude Code can be found in https://github.com/lgouger/ghostty/blob/lpg/feature/unfocused-window-opacity/notes/claude_unfocused_window_plan.md. It will be removed prior to any merging if that ever happens.

[lgouger]

From the now closed PR, #9919

The feature adds the option unfocused-window-opacity and unfocused-window-fill to dim the contents of a Ghostty window when it loses focus. If unfocused-window-opacity is not present in the config, the current behavior continues and a window losing focus does not dim. I chose to implement this feature because I found that I was starting to type, thinking I had focus, but then discovering that in many cases I did not yet have focus. This dimming when focus is lost is available in iTerm, and I've enjoyed using it there.

These changes were made with the help of Claude Code. I've reviewed and tested the code both on the mac and on an Omarchy Linux box. I've been using the MacOS version for several weeks, merging in the latest from the main ghostty-org branch each day to stay up to date.

The initial plan generated by Claude Code is in the notes directory. It's initial implementation didn't support the possibility of a fill color and made the default value for the unfocused-window-opacity 0, as a sentinel value. I manually added support for the unfocused-window-fill color (falling back to the background color if no unfocused-window-fill color defined) and made the default config value 1.0.

In the linux domain, I again worked with Claude Code on an implementation. Claude Code made code changes and responded to my feedback as I tested the resulting builds.