SwiftUI's ImageRenderer must not be called outside the main thread.
The `@MainActor` annotation is only relevant for our own code, not
for calls from frameworks. The machinations around Shortcuts end up
calling the displayRepresentation method outside the main thread.
By capturing the screenshot as NSImage, all data is retained and can
be processed outside the main thread.
This fixes an Apple Shortcuts crash for macOS 15 and earlier.
Unfortunately it looks like we can't guard these with `@available`. I'm
going to report an Apple Feedback about this but for now this gets
shortcuts working on macOS 15 and earlier.
