macOS vibrancy support?

[luciascarlet]

I wonder if it would be feasible or practical to optionally add support to use macOS's built-in "vibrancy" formulas rather than the opacity/blur combo. It definitely shouldn't replace those settings, as they for example don't allow you to use a smaller blur radius if you prefer this, but they are a noticeably different effect to alpha transparency as they also process the colours. These modes can be accessed using NSVisualEffectView.

[pluiedev]

xref #7814, #8155