Hotspot is misplaced for cursors on HiDPI when cursor created from a cairo surface

On HiDPI displays on Linux and Mac, the hotspot is misplaced and this fixes that.

Tested on Ubuntu 2021.10 and MacOS Monterey

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For the rest of this, the following MR has been put on the Gtk project: gtk!4377 (merged)

** Now patched in Gtk3 **

on macOS, the cursor scaling code creates unsightly and large cursors on retina displays, rather than creating more refined cursors.

This fixes that problem, but possibly defeats the purpose of the better images.

Here is the big cursor (sadly the screenshot utility has moved it way down the screen):

And here is the improved version:

Here is the normal mouse (for comparison):