Add config option to apply background opacity to all cell backgrounds

[thomasschafer]

Currently when background-opacity is enabled, only cells with no background have the opacity applied. Alacritty has an option transparent_background_colors, which applies the opacity to all cells. For instance, I have the following in my Tmux config which sets the background of the non-focused windows to be slightly lighter than the focused window:

setw -g window-style 'bg=#2d3049'
setw -g window-active-style 'bg=#24273A'

In Alacritty with transparent_background_colors enabled, this means that those backgrounds still have the opacity set in my config:

In Ghostty, however, there doesn't appear to be such a config option, so the windows have no such opacity:

It would be amazing if there was a similar config option in Ghostty.

[mitchellh]

#5917 accepted