Closes #15383
Closes #10993

`NSVisualEffectView` is an official API for implementing blur effects
and, by traversing the layers, we **can remove the background color**
that comes with the view. This avoids using private APIs and aligns
better with macOS’s native design.

Currently, `GPUIView` serves as the content view of the window. To add
the blurred view, `GPUIView` is downgraded to a subview of the content
view, placed at the same level as the blurred view.

Release Notes:

- Fixed the missing shadow for blurred-background windows on macOS.

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Co-authored-by: Peter Tripp <peter@zed.dev>