Setting transparency to non-zero results in title bar being completely transparent

[ychin]

Right now setting transparnecy in Vim to non-zero would cause the entire title bar to go transparent. This is due to a workaround with how view layers work but the end result looks suboptimal. We should fix it to look correct and solve the problem correctly.

See discussion here https://github.com/macvim-dev/macvim/discussions/1563#discussioncomment-12272211