Add a ScreenCaptureKit backend for macOS

[jholveck]

The macOS docs and headers indicate that CGWindowListCreateImage, the screenshot mechanism MSS uses on macOS, is now deprecated. Apple suggests using the new (as of 2022) ScreenCaptureKit instead.

I don't know offhand how widely-available ScreenCaptureKit is; it might not exist prior to macOS 13 Ventura. It also seems that the early versions were geared for streaming, not single-frame capture. Later versions added a screenshot-style API; there's some talk about the migration path in this talk.

Practically speaking, I doubt that macOS will remove CGWindowListCreateImage anytime soon; it's just too widely-used. So no urgency here.