On window resize / snap to screen edge, maintain image positioning (centering)

[JDoucette]

Feature request

I often snap the window to the left-half and right-half of the screen.
Very often to view two Aseprite instances at once.
However, I then have to work to recenter the images in both.

You can see the issue when you simply grab a window corner and resize.
The image is positioned with absolute coordinates from the top-left, not relative from the window center.

Aseprite and System version

Latest version.
Windows 11.