Aspect ratio of bitmap sprite thumbnails is not respected in Edge

[fsih]

Expected Behavior

Sprite thumbnails should have the same aspect ratio as sprites

Actual Behavior

Sprite thumbnails are stretched to a square in Edge
https://llk.github.io/scratch-gui/develop/#10123832

Steps to Reproduce

You can observe this in https://llk.github.io/scratch-gui/develop/#10123832 and other projects with bitmap costumes

Operating System and Browser

Windows 10 Edge