bug#14575: 24.3.50.1; Overlays with a window property affect other windows

[E Sabof]

Well, it's Saturday...

Btw, this bug was introduced 2 years ago, during the development of
v24.1, and no one ever said anything until now.  I guess that speaks
volumes of how frequently people use window-specific overlays.

I think the more advanced features of the display engine are generally under-used. Maybe because there aren't as many users from the front-end side of things.

Btw, I'm trying to implement variable width right/centered paragraph alignment. Now that I think about it, I might also do justified text (with varied width of spaces).

https://github.com/sabof/magic-buffer

It's still in prototype stages, but the prognosis is good. And with window-specific overlays, I can make if work even it the text is displayed in multiple windows simultaneously