Autoscale the affine transform when viewer and overlay images have different pixel sizes.

[zindy]

Hello,

this pull request is for calculating a scaling factor between viewer and overlay images if their pixel size is different, and apply it to the transform matrix.

This issue (aligning images with different pixel sizes) was discussed on the forum: https://forum.image.sc/t/align-tool-difference-in-pixel-size-for-the-fixed-and-moving-images-is-not-accounted-for/95973

Scaling can occur when a new overlay is selected and when the reset button is pressed. The new code handles both scenarios.

Cheers, Egor