Add background image support for Metal

[Rohit-Bevinahally]

• Implements Window background image configuration option #3645 for Metal.
• Uses foundation / logic from the OpenGL counterpart (Add background image support for OpenGL #4226) making changes only when necessary

[sagg0t]

Hey @Rohit-Bevinahally, thank you for this PR, I can't wait to have it merged!
While using these changes in my local build, I noticed that the background image is separate for every split. I think many ghostty users (including me) would appreciate being able to have a single image for the whole window.

What's you opinion on this? Thanks again for the PR!

P.S. also noticed that the image doesn't cover vertical padding

[jcollie]

While using these changes in my local build, I noticed that the background image is separate for every split. I think many ghostty users (including me) would appreciate being able to have a single image for the whole window.

This is because we don't have a renderer layer (Metal or OpenGL) that covers the entire window, only each individual surface. Adding that would require major surgery in how rendering is implemented and out-of-scope for this PR, and not something to be attempted without some in-depth discussion.