Do not resize macOS retina screenshots by default

[radarhere]

Resolves #9253

On macOS retina screens, ImageGrab.grab() will produce 2x images when not using bbox, but 1x images when using bbox. The issue describes this as inconsistent.

Scaling bounding box images down was added in #6152 with Pillow 9.1.0, so that is relatively new. This PR stops that resizing by default. If the user wishes to resize, they must provide a keyword-only argument,ImageGrab.grab(scale_down=True)

[hugovk]

Just a thought: rather than tacking on a bool scale_down=False, perhaps scale=2? Who knows if Apple will produce a 4x or something in the future...

[radarhere]

In main, bbox screenshots are captured at 1x.
In the current form of this PR, images are captured at 2x by default, and then 1x if scale_down=True.

If we change that to scale=2 by default, and then Apple releases 4x, users could start specifying scale=4, but our default would still be 2 for these super retina screens. So the default for 4x screens would no longer be 'full resolution' (4x) or 'bbox size' (1x), but something in the middle (2x). I don't imagine anyone would want something in the middle?

The only way I think scale is helpful is if the default is scale=1. But that's the very problem the issue creator is trying to avoid, where ImageGrab.grab(bbox) is giving 1x images currently, while ImageGrab.grab() gives 2x.

Instead, I've pushed a commit to improve the screen scaling detection, so that if Apple releases 4x in the future, the default will start capturing 4x. scale_down=True will still provide 1x.