Window background image configuration option

[mitchellh]

Discussion: #3639

Add a new configuration option: background-image which takes a file path to an image to set as the terminal background.

background-opacity should affect this.

We can add additional configurations such as whether to stretch it etc. later. For the first version, we should always stretch it to fit the window.

I suggest implementing this in the renderers rather than the apprts because it'll make it easier to make cross-platform and consistent.

[Rigo-m]

Was looking for this just right now!
Is this a "good first issue" for a zig novice?

[Obe1sance]

I just installed Ghostty and I really hope to find this feature. I've been using Tilix for the past 5 years simply because it has hands down been the best for "aesthetics". Wallpaper, wallpaper transparency, the works. Ghostty really caught my attention, time to put it to the test!.

It's my hope that if there becomes a focus on aesthetics control for aesthetics junkies like myself, that inspiration is taken from Tilix. It's truly one of a kind.