Use screencapture arguments to crop on macOS

[radarhere]

Resolves #6144

There are two ways of looking at this change.

1. Speed improvement. Rather than grabbing the complete screen on macOS, and then cropping the result to the bbox requested by the user, this PR tells screencapture the co-ordinates that we are interested in. The image does need to be resized afterwards, to avoid returning a image twice the size of the requested bbox, but presumably that is still faster than loading an image the size of the screen and cropping it.
2. Since screencapture without arguments returns an image at double resolution, cropping to the bbox can produce an unexpected offset. By passing the arguments to screencapture, Pillow can leave the problem of calculating which portion of the screen is desired to the system.